Archive for Januari, 2010

radar with Automatic Radar Plotting Aid (ARPA) capability can create tracks using radar contacts. The system can calculate the tracked object’s course, speed and closest point of approach (CPA), thereby knowing if there is a danger of collision with the other ship or landmass.

Navigation Light

The purpose of this Glossary of ECDIS-related terms is to facilitate reading and interpreting the
standards and specifications related to ECDIS issued by IMO and IHO, namely the ECDIS Performance
Standards, the IHO Special Publications S-52 (including its Appendices) and S-57.
This 3rd edition has been developed by the GlossaryWorking Group of the IHO Committee on
Hydrographic Requirements for Information Systems (CHRIS). It has been completely revised based on the
latest editions of the relevant ECDIS-related standards and specifications.
The reader should be aware that it is not intended to cover everything that has a bearing on ECDIS,
and that many definitions only reflect a rather narrowmeaning the terms have in conjunction with ECDIS.
The Glossary, therefore, should not be used as general reference for hydrographic, navigational or
information technology terms. Some of the relevant IHO specifications contain a glossary of terms on their
own. Only those definitions which have a wider use beyond the limits of a particular publication have been
incorporated from these specific glossaries.
The “invention” of new definitions has been avoided as far as possible. Use has beenmade instead
of either general dictionaries such as the Hydrographic Dictionary (IHO Special Publication S-32), and
Bowditch, or of other relevant documents. References are provided in square brackets. It should be noted
that sometimes a definition taken from another sourcemay be an excerpt from the original definition, or is
slightlymodified in order to better meet ECDIS terminology.Where it was felt, however, that the special
meaning of a term in conjunction with ECDIS requires a correspondingly specialized approach, new
definitions were developed.
Advantage has been taken of guidelines provided in the “Short guide for writing definitions”
published by the ISO TC211 Committee [ISO]. In particular, an attempt has been made to separate the
core of the definition from any additional explanations, examples etc. through the addition of Notes.
All entries to the Glossary are given in lower case, except acronyms and proper names. Cross
references to other definitions in the Glossary are provided by italicizing the respective term. Acronyms are
integrated in the Glossary with cross references. Some illustrations are annexed to the Glossary to show
relationships between terms.
Any suggestions for improvement, corrections and extensions are welcome. As theWorkingGroup
will be disbanded on publication of this edition, comments should be addressed to the Chairman of CHRIS
through the International Hydrographic Bureau.

The extent to which a measured or enumerated value agrees with the assumed or accepted value.
See precision. [HD]
aid to navigation
Visual, acoustical or radio device external to a vessel designed to assist in the determination of a
safe course or of a vessel’s position, or to warn of dangers and obstructions. [Bowditch]
See Automatic Identification System.
An alarm or alarm system which announces by audible means, or audible and visual means, a
condition requiring attention. [PS]
all other information
Term used in the Performance Standards for ECDIS to describe information not belonging to the
standard display. Also sometimes called “on-demand information”. (See Fig. 3)
application profile
Data structure. A defined subject of the S-57. [TS]
Note: An application profile is defined for a specific purpose, such as the transfer of ENC data.
An entity controlling the application of the update information, e.g. the mariner keying in update
information, or the software inside ECDIS automatically processing the ENC update information.
The 2-dimensional geometric primitive of an object that specifies location. (See Fig. 4)
See Automatic Radar Plotting Aid.
A characteristic of an object. It is implemented by a defined attribute label/code, acronym,
definition and applicable values. In the data structure, the attribute is identified by its label/code.
The acronym is only used as a quick reference in related documents and in ENC Product
Specification. Attributes are either qualitative or quantitative. [TS]
Note: see Appendix A, IHO Object Catalogue.
attribute label/code
A fixed length numeric label or a 2-byte unsigned integer code of an attribute as defined in
Appendix A, IHO Object Catalogue. [TS]
attribute value
See attribute label/code.
Automatic Identification System (AIS)
An automatic communication and identification system intended to improve the safety of
navigation by assisting in the efficient operation of vessel traffic services (VTS), ship reporting,
ship-to-ship and ship-to-shore operations.
Note: A transponder is an example of an AIS. AIS information may be displayed on an ECDIS.

automatic updating
Either the semi-automatic or the fully automatic means of updating the ENC/SENC. [UG]
Automatic Radar Plotting Aid (ARPA)
A system wherein radar targets are automatically acquired and tracked and collision situations
computer assessed and warnings given.
The bearing of a geographical position, measured clockwise from north through 360 degrees.
back-up arrangement
Facilities enabling safe take-over of the ECDIS functions and measures facilitatingmeans for safe
navigation of the remaining part of the voyage in case of an ECDIS failure. [PS]
base data
S-57 conforming data at the data producer’s site that does not contain any update records. Once
this data is exchanged it becomes target data at the applier’s site. [TS]
bearing (BRG)
The direction from a reference station, usually from 000 degrees at the reference direction,
clockwise through 360 degrees. [Bowditch]
block correction
see chartlet.
cartographic object
Feature object which contains information about the cartographic representation (including text of
real world entities. [TS]
Note: The ENC Product Specification prohibits the use of cartographic objects in ENCs.
cautionary note
Information calling special attention to some fact, usually a danger area shown on a chart, or other
publication. [HD]
The basic unit for the distribution of ENC data covering a defined geographical area bounded by
two meridians and two parallels, the content of which must not exceed 5 Mbytes, and which is
intended for a particular navigational purpose.
A sequence of one or more edges.
chain node
Data structure in which the geometry is described in terms of edges, isolated nodes and connected
nodes. Edges and connected nodes are topologically linked. Nodes are explicitly coded in the data
structure (see Fig. 5). [TS]
Note: Spatial objects may be shared between feature objects. Where spatial objects are shared,
lines that cross are broken (into edges) at the intersection and a logical node is implied (ie nodes
are not explicitly created, the geometry of the node remains part of the edge). Where spatial
objects are not shared, lines may cross without being broken at the intersection. Areas are defined
by the sequence of edges that comprise their boundaries. Lines are comprised of edges. Point
feature objects may only reference isolated nodes.

chart amendment patch
see chartlet.
chart cell
see cell.
chart information
HO-information relevant to navigation contained in nautical charts. (See Fig. 2)
chart datum
A permanently established surface fromwhich soundings or tide heights are referenced. Short for
chart sounding datum. [HD]
Note: A plane so low that the tide will not frequently fall below it. [TR]
A chart specifically designed to meet the requirements of marine navigation, showing depths of
water, nature of bottom, elevations, configuration and characteristics of coast, dangers and aids to
navigation. [HD]
Note: The carriage of up-to-date charts (plus certain other nautical publications) by vessels at sea
is a mandatory requirement of SOLAS regulation V 20. The term nautical chart may be applied
also to a specially compiled data base (e.g., the ENC), from which such a map can be displayed.
chart symbol
A character, letter, line style, or similar graphic representation used on a chart to indicate some
object, characteristic, etc. [HD]
All navigational procedures carried out on a chart, the elements and parameters ofwhich are listed
in Annex 3 of the ECDIS Performance Standards.
See Committee on Hydrographic Requirements for Information Systems.
C.I.E. colour system
Colour specification system established by the Commission Internationale de l’Eclairage, which
permits a replicable description of any colour on any equipment; in contrast with other colour
schemes, which are apparatus specific. Colour is usually expressed in terms of the x and y
chromaticity coordinates for the widely used chromaticity diagram, and a third dimension Y
representing the luminance (perceived as brightness) of the light in candela per square metre
(cd/m2). C.I.E. colour coordinates can be transformed into RGB coordinates of a calibrated CRT.
clarifications section
One of the mechanisms used to maintain S-57. See maintenance document.
Excess information or noise data on a display reducing its readability.
Note: E.g., information overload on a chart, or confusing unwanted wave echoes on a radar
display. [HD]
coastal warning
A navigational warning promulgated by a national co-ordinator covering a coastal region or a
portion thereof.

collection object
A feature object which describes the relationship between other objects. [TS]
Note: An example of a collection object in S-57 is “aggregation” which is used, for example, to
group together the different objects which together constitute a Traffic Separation Scheme.
colour calibration
In order to reproduce the IHO colours for ECDIS, a colour calibration at the monitor must be
performed to transform the CIE-specified colours for ECDIS into the colour coordinate systemof
the screen. Calibration will ensure correct colour transfer at the time a display leaves the
manufacturer’s plant. Methods of colour maintenance under operational use are under
Note: colour calibration is described in Appendix 2 of IHO S-52.
colour differentiation test diagrams
Screen diagrams supplied in the Presentation Library for use by the mariner to check brightness
and contrast settings and to find out whether the screen still has the capability of distinguishing the
important colours.
colour fill
Method of distinguishing different area features by filling areas with colour. “Transparent” colour
fill is used to allow information to show through the fill, e.g., soundings in a traffic separation
colour tables
Five colour tables are supplied in IHO S-52 Appendix 2, for selection by the mariner. They are
bright sun, day white-background, day black-background, dusk and night.
colours for ECDIS
The colours specified by the IHO for ECDIS have been developed by Perception Institutes to give
a clear display through the optimum colour contrast under all viewing conditions, and tominimise
light emission on the darkened bridge at night. Colours are used to aid in distinguishing between
features. Colours for ECDIS are specified in the CIE colour system.
Committee on ECDIS (COE)
Committee of the IHO responsible for ECDISmatters until 1995. Nowreplaced by the Committee
on Hydrographic Requirements for Information Systems (CHRIS).
Committee on Exchange of Digital Data (CEDD)
Committee of the IHO responsible for the Transfer Standard for Digital Hydrographic Data until
1995. Now replaced by the Committee on Hydrographic Requirements for Information Systems
Committee on Hydrographic Requirements for Information Systems (CHRIS)
A Committee of the IHO tasked with promoting and coordinating the development of official
digital products and services.
In cartography, the selection, assembly, and graphic presentation of all relevant information
required for the preparation of a newmap/chart or a new edition thereof. Such informationmay be
derived from other maps/charts, aerial photographs, surveys, new data, and other sources.
compilation scale
The scale at which the ENC data was compiled.

compilation update
The correction information which has been issued since the last new edition of the ENC or since
the last official update applied to the SENC, compiled into a single, comprehensive ENC update.
Note: Example: if a buoy has been relocated two or more times since the ENC Edition, only the
last position, tailored to correct the position contained in the ENC Edition as issued, would be
included. Thus, the application of a compilation update is to correct the effective ENC Edition at
the time of its distribution to a user, which could be some months or years after the Edition’s initial
distribution. Compilation update is implemented in S-57 by means of a re-issue of an ENC.
conditional symbology procedure
A decision-making procedure used to link an object-class to a symbol in cases where the symbol
depends on circumstances (e.g. whether a wreck is symbolised as an “isolated danger” depends on
its relationship to the safety contour chosen by the mariner), or where symbolisation is complex
(e.g. light sectors).
connected node
A node referred to as a beginning and/or end node by one or more edge. Connected nodes are
defined only in the chain-node, planar graph and full topology data structures. [TS]
correction information
See update information.
See update information.
corrections section
One of the mechanisms used to maintain S-57. See maintenance document.
corrupted data
Any change in data introduced during, and as a result of, its transmission. [UG]
The horizontal direction in which a vessel is intended to be steered, expressed as an angular
distance from north clockwise through 360 degrees. [Bowditch]
course made good (CMG)
The single resultant direction from a vessel’s point of departure to its point of arrival at any given
time. [Bowditch]
Note: Relates to each leg of the track.
course over ground (COG)
The direction of a vessel’s path actually followed, usually a somewhat irregular line. [Bowditch]
course up display
The information is shown on the display (radar or ECDIS) with the direction of the vessel’s course
upward. The display orientation is stabilized by means of the gyro until a new course direction is
fed in.
Note: Course-up orientation avoids the disadvantages with the head-up display due to yawing or
other changes in heading.
cumulative update
The collection of all sequential correction information which has been issued since the last new
edition of the ENC or since the last official update applied to the SENC. [UG]

The process of querying a point-symbol, line or area for further information from the data base
which is not represented by the symbol.
data dictionary
Conveys the meaning of entities and attributes, the relationship between entities and attributes and
the relationship between attribute and value domains.
data model
A conceptual specification of the sets of components and the relationships among the components
pertaining to the specific phenomena defined by themodel reality.Adatamodel is independent of
specific systems or data structures. The S-57 data model defines real world entities as a
combination of descriptive and spatial characteristics. These characteristics are defined in terms of
feature objects and spatial objects and the relationship between them. See Fig. 4.
data quality indicator
Indication of reliability and accuracy of surveys of a particular area provided through CATZOC
attribute of the quality of data meta object in S-57.
data set
A logical grouping of S-57 data to which the S-57 data set descriptive records apply. The data set
descriptive records contain meta data. The use of data set descriptive records is product specific
and is, therefore, defined by a product specification. If the data set descriptive records are repeated
for each file in an exchange set, an instance of a file containing the data set descriptive records is
called a data set. If the data set descriptive records are encoded generally for the whole exchange
set, the exchange set is referred to as a data set. [TS]
data structure
A computer interpretable format used for storing, accessing, transferring and archiving data. [ISO]
datum (geodetic)
A set of parameters specifying the reference surface or the reference coordinate system used for
geodetic control in the calculation of coordinates of points on the earth. Commonly datums are
defined as horizontal and vertical datums separately.
For a local geodetic datum the reference surface is defined by five parameters: the latitude and
longitude of an initial point, the azimuth of a line from this point, and the parameters of the
reference spheroid.
Absolute datums specify the initial point of the reference ellipsoid to be (ideally) located at the
earth’s centre of mass. For modern reference systems using datum information given by satellites
additional parameters are defined, e.g. gravity models. [HD]
datum (vertical)
Any level surface (e.g. sea mean sea level) taken as a surface of reference from which to reckon
elevations. [HD]
Note: See also chart datum.
The deterioration in the performance of a system below a specified minimum level.
Note: For example, the reduction in the information content of a display, a data base or other
means of providing information below an acceptable or agreed level. [IEC]

Differential GPS (DGPS)
see GPS
Differential System
An augmentation system whereby radionavigation signals aremonitored at a known position and
the corrections so determined are transmitted to users in the coverage area. [GNSS]
digital terrain model (DTM)
A model representing the terrain surface by a collection of digital data,mainly coordinates in three
dimensions, x, y, and z, and a digitally defined method to interpolate arbitrary terrain heights in
between the stored values. [HD]
Note: Not to be confused with digital cartographic model which may also contain digital data
representing terrain, but in a form suitable for the drawing of a particular kind of map.
digitizing conventions
see encoding conventions
A visual presentation of data.
Note: For example, a presentation of a line of alphanumeric data, a window or the entire screen.
display base
see display category
display category
The ECDIS Performance Standards establish three categories for SENC objects:
display base: permanently retained on the display.
standard display: displayed at switch-on, recalled by single operator action.
all other information: displayed individually (by class) on demand.
The category of every object is given in the look-up table. See Fig. 2, 3.
display generator
Software written by the ECDIS manufacturer to take an object from the SENC, assign a symbol
and colour to it, and present it appropriately on the display, using the tools and procedures
provided in the Presentation Library.
display priority
Detailed rules to decide which line or point symbol is to be shown when two objects overlap.
Priority 2 overwrites 1. Display priority is given in the look-up table.
display priority for radar
Radar is normally written over most chart areas and under most lines and point symbols; the
detailed priorities are given in the look-up table.
display priority layer
Layers to establish the priority of information on the display. Information of lower priority must
not obscure higher priority information.
display scale
The ratio between a distance on the display and a distance on the ground, normalised and
expressed as for example 1/10,000 or 1:10,000.
drawing sequence
The implementation of display priority.

Note: Specifies, within a display priority layer, which of two overlapping objects is drawn on top
of the other.
The name given to the data structure in S-57 versions 1 and 2. It is not used in S-57 edition 3.
ECDIS Chart 1
An ECDIS version of IHO INT1, including all symbols, linestyles and colour coding used for chart
and navigation symbols, contained in the Presentation Library.
A one-dimensional spatial object, located by two or more coordinate pairs (or two connected
nodes) and optional interpolation parameters. If the parameters are missing, the interpolation is
defaulted to straight line segments between the coordinate pairs. In the chain-node, planar graph
and full topology data structures, an edge must reference a connected node at both ends and must
not reference any other nodes. [TS]
electronic chart
Very broad termto describe the data, the software, and the electronic system, capable of displaying
chart information. An electronic chart may or may not be equivalent to the paper chart required by
Electronic Chart Data Base (ECDB)
Themaster data base for Electronic Navigational Chart Data (ENCD), held in digital form by the
national hydrographic authority, containing chart information and other nautical and hydrographic
information. (See Fig. 1). [UG]
Electronic Chart Display and Information System (ECDIS)
A navigation information system which with adequate back-up arrangements can be accepted as
complying with the up-to-date chart required by regulation V/20 of the 1974 SOLAS Convention,
by displaying selected information from a System Electronic Navigational Chart (SENC) with
positional information from navigation sensors to assist the mariner in route planning and route
monitoring, and if required display additional navigation-related information. [PS]
Electronic Chart Systems (ECS)
Generic term for equipment which displays chart data but which is not intended to comply with
the IMO Performance Standards for ECDIS, and is not intended to satisfy the SOLAS Chapter V
requirement to carry a navigational chart.
Electronic Navigational Chart (ENC)
The data base, standardized as to content, structure and format, issued for use with ECDIS on the
authority of government authorized hydrographic offices. The ENC contains all the chart
information necessary for safe navigation and may contain supplementary information in addition
to that contained in the paper chart (eg sailing directions) which may be considered necessary for
safe navigation. (See Fig. 1, 2). [PS]
Electronic Navigational Chart Data (ENCD)
The national data for an Electronic Navigational Chart (ENC) in a format acceptable to an ENC
Coordinator. (See Fig. 1). [UG]
Note: The content, structure and format of the ENC are specified in IHO S-57 edition 3 and the
associated ENC product specification [IEC].

Electronic Navigational Chart Data Base (ENCDB)
The master data base for production and maintenance of the ENC, compiled from national ENC
data (ENCD). (See Fig. 1). [UG]
See Electronic Navigational Chart.
ENC cell structure
See cell.
ENC data
See Electronic Navigational Chart Data (ENCD).
ENC Product Specification
Appendix B1 of S-57 which specifies the content, structure and other mandatory aspects of an
ENC test data set
A standardized data set supplied on behalf of the International HydrographicOrganization (IHO)
that is necessary to accomplish all IEC testing requirements for ECDIS. This data set is encoded
according to the IHO S-57 ENC product specification and contains update information based on
IHO S-52 Appendix 1. The specific requirements are listed in Annex F of IEC 61174.
The identification of fields and records and the grouping of fields and records and the data syntax
rules used.
encoding conventions
A set of rules which must be followed when encoding data for a particular purpose.
Note: For example, S-57 Appendix B1 lists the encoding conventions to be used when encoding
ENC data, such as the rules to be followed when encoding depth areas. Sometimes referred to as
digitizing conventions.
Enhanced Group Call (EGC)
A global automatic service by INMARSAT for addressing commercial messages (FleetNet) or
Maritime Safety Information (MSI) (SafetyNet) to groups or ships or all vessels in both fixed and
variable geographical areas. Data transmission is based on INMARSAT Standard-C and takes
place at an effective transmission rate of 600 bit/s. [UG]
Any concrete or abstract thing of interest, including associations of things. [ISO]
exchange format
A specification for the structure and organization of data to facilitate exchange between computer
exchange set
The set of files representing a complete, single purpose (i.e. product specific) data transfer. The
ENC Product Specification defines an exchange set which contains one Catalogue file and at least
one data set file. [TS]

extensions document
One of the three documents used to maintain S-57. It contains extensions, or other significant
changes to the Standard, which have been agreed by the appropriate IHO committee or working
group, and will be included in the next edition of the Standard.
See also clarifications document, corrections document.
Note: Available from IHB only on demand. [TS]
A two dimensional spatial object. A face is a continuous area defined by a loop of one or more
edges which bound it. A facemay contain interior holes, defined by closing loops of edges. These
interior boundariesmust be within the outer boundary.No boundarymay cross itself or touch itself
other than at the beginning/end node. None of the boundaries may touch or cross any other
boundary. Faces are defined only in the full topology data structure. [TS]
Representation of a real world phenomenon. [ISO]
Note: For example, a particular cardinal buoy represented through a symbol on a chart.
feature object
An object which contains the non-locational information about realworld entities. Feature objects
are defined in Appendix A, IHO Object Catalogue. See also geo, meta, collection and
cartographic objects. See Fig. 4. [TS].
feature record
A feature record is the implemented term used in the S-57 data structure for a feature object (i.e. a
feature object as defined in the data model is encoded as a feature record in the data structure).
There are four types of feature records: geo, meta, collection and cartographic. [TS]
A named collection of labelled subfield(s). For example, IHO attribute label/code and IHO
attribute value are collected into a field named Feature Record Attribute.
An identified set of S-57 records collected together for a specific purpose. The file content and
structure must be defined by a product specification. [TS]
fully automatic updating
The application of corrections to ENC data in the SENC in a fully integrated state,without human
intervention at the receiving end. [UG]
full topology
A 2-dimensional data structure in which the geometry is described in terms of nodes, edges and
faces which are all topologically linked. A planar graph with faces. [TS]
The omission of less important detail when compiling a chart. Its purpose is to avoid overloading
charts where space is limited. [HD]
geo object
A feature object which carries the descriptive characteristics of a real world entity. [TS]
Note: The positional information is provided through the spatial object.

Geographic Information System (GIS)
A computer based system for handling and integrating data from a variety of sources which is
directly or indirectly spatially referenced to Earth. [HD]
geometric primitive
One of the three basic geometric units of representation: point, line and area. (See Fig. 4). [TS]
Global Maritime Distress and Safety System (GMDSS)
A global communications service based upon automated systems, both satellite based and
terrestrial, to provide distress alerting and promulgation of maritime safety information to
mariners. [HD]
Note: This system has been developed by IMO through the SOLAS Convention.
Global Navigation Satellite System (GNSS)
A world-wide position, time and velocity radiodetermination systemcomprising space, ground and
user segments of which GPS and GLONASS are components. [GNSS]
GLONASS (Global Navigation Satellite System)
A space-based, radio-positioning, navigation and time-transfer systemoperated by the Government
of the Russian Federation. GLONASS to which differential corrections have been applied is
known as Differential GLONASS (DGLONASS).
GPS (Global Positioning System)
A space-based, radio-positioning, navigation and time-transfer system operated by the United
States Government. GPS to which differential corrections have been applied is known as
Differential GPS (DGPS).
ground stabilization
A display whereby own ship position is referenced to the ground. Usually performed in
conjunction with radar/ARPA, it can be determined by computing set and drift or by the use of
GPS/differential GPS.
Harmonization Group on ECDIS (HGE)
Joint Working Group of IMO and IHO tasked with maintaining the ECDIS Performance
Standards and addressing other related issues.
heading (HDG)
The direction in which a vessel is pointed, expressed as an angular distance from north clockwise
through 360 degrees. A constantly changing value as a vessel yaws back and forth across the
course due to the effects of sea, wind, etc. [Bowditch]
head-up display
The information shown on the display (radar or ECDIS) is directed so that the vessel’s heading is
always pointing upward. This orientation corresponds to the visual view from the bridge in
direction of the ship’s heading. This orientation may require frequent rotations of the display
contents. Changing the ship’s course, or yawing of the vessel may render this unstabilized
orientation mode unreadable (See course-up display).
Information content of the SENC originated by hydrographic offices. It consists of the ENC
content and updates to it. (See Fig.2)
See object identifier.

See International Electrotechnical Commission.
Specification of symbols, abbreviations and terms to be used in the International Chart Series of
IHO Transfer Standard for Digital Hydrographic Data
Originally published as SP57 (later S-57) Version 1 and then Version 2. The latest release of the
standard, S-57 Edition 3, consists of a TheoreticalDataModel, Data Structure, Object Catalogue,
ENC Product Specification, Use of the Object Catalogue for ENC and an Object Catalogue Data
Dictionary Product Specification.
IHO test data set
See ENC test data set.
See indicator.
Visual indication giving information about the condition of a system or equipment. [PS]
information from other sources
Information contained within SENC not originating from HO’s or the mariner.
Note: E.g., manufacturer’s chart data of areas where no HO-information is available. (See Fig. 2,
International Maritime Satellite Organization, an international consortium which provides
worldwide satellite communications links to ships at sea.
See IHO INT 1.
International Electrotechnical Commission (IEC)
A worldwide non-governmental organization for standardization comprising all national
electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international cooperation on all questions concerning standardization in the electrical and
electronic fields. [IEC]
Note: Committee 80, Working Group 7 of IEC is responsible for developing the Performance
Requirements for ECDIS to be published as document IEC 61174.
International Hydrographic Organization (IHO)
Coordinates the activities of national hydrographic offices; promotes standards and provides
advice to developing countries in the fields of hydrographic surveying and production of nautical
charts and publications. [HD]
International Maritime Organization (IMO)
The specialized agency of the United Nations responsible for measures to improve the safety of
international shipping and to prevent marine pollution from ships [IMO].
ISO 8211
ISO standard on “Information processing -Specification for a data descriptive file for information
interchange”. This international standard specifies an interchange mechanism to facilitate the
transfer of files containing data records between computer systems. It defines a generalized
structure which can be used to transmit, between systems, records containing a wide variety of
data types and structures. It provides the means for the description of the contents of data records
but does not define the contents.
ISO 10646
This ISO standard specifies the Universal Multiple-Octet Coded Character Set (UCS). It is
applicable to the representation, transmission, interchange, processing, storage, input and
presentation of the written form of the languages (scripts) of the world as well as additional
Note: used to represent certain national language character sets, for example Arabic and Chinese,
in S-57.
isolated node
An isolated zero-dimensional spatial object that represents the geometric location of a point
feature. An isolated node is never used as a beginning or end node. [TS]
Issuing Authority
The official agency which issues the ENC and ENC updates. [UG] See Regional ENC
Coordinating Centre.
An identifier which establishes linkages, e.g. between different layers, or features and attributes.
See attribute label/code.
A group of related information displayed as a whole.
A line connecting two waypoints.
The one-dimensional geometric primitive of an object that specifies location. (See Fig. 4)
List of Lights
A publication tabulating navigational lights, with their locations, candle powers, characteristics,
etc. to assist in their identification, and details of any accompanying fog signal. A list of lights
may contain other information useful to a navigator. Also called light list. [HD]
Note: This publication is issued under the authority of a marine administration.
List of Radio Signals
A publication tabulating and combining particulars of: coast radio stations, port radio stations,
radio direction finding stations, radiobeacons, etc., as well as other information on radio services
useful to a navigator. [HD]
Note: This publication is issued under the authority of a marine administration.
local datum
Any geodetic datum defined for national purposes other than WGS84 which is adopted for use
with ECDIS.

local updates
A generic termto indicate all update information other than official updates, regardless of source;
for application as a manual update only. [UG]
log file
A record of updates, including time of application and identification parameters described in
Appendix 1 of S-52. [IEC]
look-up table
A table giving symbology instructions to link SENC objects to point, line or area symbolisation,
and providing display priority, radar priority, IMO category and optional viewing group.
maintenance document
S-57 (apart from Annex A of Appendix B1: “Use of the Object Catalogue”) is maintained by
means of numbered Maintenance Documents which are produced following each meeting of the
Transfer StandardMaintenance and Application DevelopmentWorking Group (TSMAD). These
documents are distributed with the Standard and their contents are also available on the IHB’s
Bulletin Board System and on the IHO Web site. Each Maintenance Document contains the
following three sections:
Clarifications. This section contains improvements to the wording of the Standard. These are
editorial amendments which do not result in any substantive change to the Standard.
Corrections. This section contains substantive changes to the Standard to correct factual errors or
amend the contents of the Standard. These changes will be only included in the next edition (Ed 4.0)
of the Standard.
Extensions. This section contains extensions, or other significant changes to the Standard, which
have been agreed by the appropriate IHO committee or working group and will be included in the
next edition (Ed 4.0) of the Standard.
manual updating
The manual application of corrections to ENC data in the SENC by a human operator, usually
based on unformatted update information (such as printed NtMs, voice radio, verbal
communication etc.). [UG]
Note: In order for the SENC to accept manual updates, the update information must be entered in
a structured way at least compatible with the relevant ECDIS standards.
mariner’s information
Information originated by and entered by the mariner to the SENC, e.g. area of strong currents.
(See Fig. 2, 3)
mariner’s navigational objects
Features, other than chart objects, such as the ownship symbol and velocity vector, planned route,
electronic bearing lines, etc from Appendix 3 of the Performance Standards for ECDIS, which are
specified in IEC document 61174.
Maritime Safety Information (MSI)
Navigational and meteorological warnings, meteorological forecasts, distress alerts and other
urgent safety related messages broadcast to ships. [HD]
Note: ECDIS update information is considered MSI.

An array of regularly spaced locations. [TS]
Note: For future use.
meta object
A feature object which contains information about other objects. [TS]
Note: For example compilation scale or vertical datum.
nautical chart
See chart.
A geographical sea area established for the purpose of co-ordinating the transmission of radio
navigational warnings.
Navarea warning
A navigational warning issued by the Navarea co-ordinator for the Navarea.
navigational aid
An instrument, device, chart, method, etc., used onboard intended to assist in the navigation of a
vessel. [HD]
navigational chart
Same as nautical chart (in the context of ECDIS).
navigational information
The information contained in mariner’s navigational objects.
navigational purpose
The specific purpose for which a chart cell has been compiled. There are six such purposes,
namely berthing, harbour, approach, coastal, general and overview.
navigational symbol
see non-chart symbol.
navigational warning
A broadcast message containing urgent information relevant to safe navigation.
Note: See local updates, Navarea warning.
Narrow-band direct-printing telegraphy system for transmission of navigational and
meteorological warnings and urgent information to ships.
A zero-dimensional spatial object, located by a coordinate pair. A node is either isolated or
connected. (See Fig. 4) [TS]
non-chart symbol
A symbol for information such as own ship’s position, course made good, etc., which appears on
the ECDIS but which does not appear on the printed chart. See Mariners Navigational Objects.
non-HO information
Information contained in the SENC provided by non-HOsources (mariner’s information or other
sources outside HOs). (See Fig. 2, 3)

north-up display
Information shown on the display (radar or ECDIS) with the north direction upward.
Note: The north-up display corresponds with the usual orientation of the nautical chart.
Notice to Mariners (NtM)
A periodical notice issued bymaritime administrations, or other competent authorities, regarding
changes in aids to navigation, dangers to navigation, important newsoundings, and, in general, all
such information as affects nautical charts, sailing directions, light lists and other nautical
publications. [HD]
See Notice to Mariners.
An identifiable set of information. An object may have attributes and may be related to other
objects. Also see spatial object and feature object. [TS]
Object Catalogue
The Object Catalogue is the feature schema for S-57. Its primary function is to provide a
description of real world entities. It contains a list of feature object classes (each relating to a real
world entity), attributes and allowable attribute values.
object class
A generic description of objects which have the same characteristics.
Note: Examples of object classes in S-57 are “buoy, cardinal” and “caution area”.
object description
The definition of which object class a specific object belongs to.
object identifier
The identification of a S-57 feature object. The object identifier is the concatenation of the
“Producing Agency”, “Feature Identification Number” and “Feature Identification Subdivision”
subfields. Within the context of this Standard the object identifier is referred to as the “Long
Name”. [TS]
Note: This provides a unique world-wide identifier for any object as specified in Para 3.1 of the
ENC Product Specification.
official HO data
See HO information.
official updates
Updates provided in digital format by the Issuing Authority of the ENC being corrected, for
integration with the ENC data in the SENC. [UG]
on-demand information
The SENC information which is not part of the standard display. See also: All other information.
See Fig. 2, 3.
The mode in which information on the ECDIS is being presented. Typical modes include:
north-up. – as shown on a nautical chart, north is at the top of the display.
ship’s head-up. – based on the actual heading of the ship (e.g. ship’s gyrocompass).
course-up display. – based on the course or route being taken.

other chart information
See display category and Fig 3.
other navigational information
Navigational information not contained in the SENC, thatmay be displayed by an ECDIS, such as
radar information. (See Fig. 2)
To display the chart information at a display scale larger than the compilation scale. Overscaling
may arise from a deliberate overscaling by the mariner, or from automatic overscaling by ECDIS
in compiling a display when the data included is of various navigational purposes.
Note: An indication is provided only for the partial overscale area of the display.
overscale area
When the data displayed is from data of two different navigational purposes the chart displaywill,
where drawn at the larger scale, include an overscale area of data from the smaller scale cell in
order to complete the display. This area should be identified by the “overscale pattern” of the
Presentation Library.
Note: E.g., when sailing across the junction between a harbour chart and an approach chart.
own ship’s safety contour
The contour related to the own ship selected by the mariner from the contours provided for in the
SENC, to be used by ECDIS to distinguish on the display between the safe and the unsafe water,
and for generating anti-grounding alarms.
own ship’s symbol
A non-chart symbol used in ARPA and ECDIS to show the ship’s position on the chart or ARPA
The term which identifies the vessel upon which an ECDIS is operating.
Performance Standards for ECDIS
Minimum performance requirements for ECDIS, adopted by IMO 23 November 1995 as
Assembly resolution and published as Annex to IMO Resolution A19/Res 817 (15 December
Contraction for “picture element”. The smallest element resolvable by electronic raster devices
such as scanner, display, and plotter. [HD]
planar graph
A 2-dimensional data structure in which the geometry is described in terms of nodes and edges
which are topologically linked. A special case of a chain-node data structure in which edgesmust
not cross. Connected nodes are formed at all points where edges meet. (See Fig. 5) [TS]
The 0-dimensional geometric primitive of an object that specifies location. (See Fig. 4)
A non-selfintersecting, closed chain defining the boundary of an area. [HD]

product specification
A defined subset of the entire specification combined with rules, tailored to the intended usage of
the transfer data. See ENC product specification. [TS]
The degree of refinement of a value. Not to be confused with accuracy. [HD]
Cartographic design including drawing, use of symbols, use of colours, use of conventional practices,
Presentation Library
A set of mostly digital specifications, composed of symbol libraries, colour schemes, look-up
tables and rules, linking every object class and attribute of the SENC to the appropriate
presentation of the ECDIS display. Published by IHO as Appendix 2 of S-52.
A regular array with information pertaining to each element (pixel) or group of elements. [TS]
raster data presentation
Method of representing all, or part, of a chart digitally by a matrix-like scheme of pixels. [HD]
A S-57 construct which is comprised of one or more tagged S-57 fields and identified by a key.
Regional ENC Coordinating Centre (RENC)
In the Worldwide Electronic Navigational Chart Data Base (WEND) the IHO network of
Hydrographic Offices, the regional node being responsible, as Issuing Authority, for official ENCs
and official updates compiled from national ENC data.
A logical link between two elements from the data model which may be spatial (e.g. topological
relationship) and/or non-spatial. In general a relationship is implemented in the data structure as a
Note: The relationships permitted in ENC data are described in the ENC Product Specification.
relative motion display
A display in which own ship remains stationary, while all other charted information and targets
move relative to own ship’s position. See also true motion display.
See Regional ENC Coordinating Centre.
Capability of depicting detail, represented by the smallest distance apart at which two objects can
be seen to be separate. Depends on pixel size, i.e. screen dimension divided by number of pixels.
A sequence of waypoints and legs.

route monitoring
The operational navigational ECDIS function in which the chart information is displayed, under
control of the positioning sensor input, according to the vessel’s present position (either in true
motion or relative motion display mode).
route planning
The pre-determination of course, speed, waypoints and radius in relation to the waters to be
navigated, and in relation to other relevant information and conditions. [NAV]
Note: See also leg, route, voyage plan, waypoint.
IHO Special Publication 52, titled “Specifications for chart content and display aspects of ECDIS”.
Additional specifications and guidance are developed in the Appendices of S-52.
IHO Special Publication 57. See IHO Transfer Standard for Digital Hydrographic Data.
safety contour
See Own Ship’s Safety Contour.
safety depth
The depth defined by the mariner, e.g. the ship’s draft plus underkeel clearance, to be used by the
ECDIS to emphasize soundings on the display equal to or less than this value.
A service provided through INMARSAT’s Enhanced Group Calls (EGC) system which will be
used by Administrations for the promulgation of Maritime Safety Information, such as Navarea
and stormwarnings, shore-to-ship distress alerts and routineweather forecasts to the high seas and
those coastal waters not served by Navtex. IHO proposes to include promulgation of electronic
chart corrections as well. [UG]
sailing directions
A publication issued under the authority of a marine administration providing general coastal
navigation information such as aids to navigation, harbour approaches and facilities, and other
details necessary which it may not be feasible to show on the corresponding nautical charts.
Note: Sometimes referred to as Coastal Pilots or Coast Pilots.
The ratio between the linear dimensions of a chart, map, drawing, etc., and the actual dimensions
represented. [HD]
scale bar
A vertical bar scale of 1 nautical mile divided into 1/10 ths., intended to convey an immediate
sense of distance. Replaced at display scales smaller than 1/80,000 by a 5-mile latitude scale.
The face of a CRT or other graphic device used to display information from a computer.
semi-automatic updating
The application of corrections to ENC data in the SENC Updating in a fully integrated state, by
hard media or telecommunications transfer in amanner which requires human intervention at the
ECDIS interface.

See Systems Electronic Navigational Chart.
simplified symbols
Symbols designed specifically for ECDIS, for fast draw and to give the maximumclarity under all
conditions of viewing the CRT. They are less complex than the equivalent paper chart symbols.
Note: For example, the simplified buoy and beacon symbols are solid, colour-filled symbols
designed to show up better at night or in bright sun conditions than the true-to-shape paper chart
symbols. The mariner can choose paper chart or simplified symbol sets.
International Convention for the Safety of Life at Sea developed by IMO. The contracting
governments undertake to promulgate all laws, decrees, orders and regulations and to take all other
steps which may be necessary to give the present Convention full and complete effect, so as to
ensure that, from the point of view of safety of life, a ship is fit for the service for which it is
intended [SOLAS].
sounding datum
Same as chart datum (for the purposes of ECDIS).
spaghetti data
A data structure in which all lines and points are unrelated to each other (i.e. no topological
relationships exist in the data structure). [TS]
Note: This data structure is not permitted for ENC.
spatial object
An object which contains locational information about real world entities. See Fig. 4. [TS]
Note: for example, in S-57 the location of a buoy or the boundary of a caution area.
spatial record
A spatial record is the implemented term used in the S-57 data structure for a spatial object (i.e. a
spatial object as defined in the data model is encoded as a spatial record in the data structure).
There are three types of spatial records: vector, raster and matrix. [TS]
In general, the rate of motion or distance per unit of time. When used in conjunction with ECDIS,
speed could be either: [Bowditch]
speed of advance (SOA) – speed intended to be made along the track
speed over ground (SOG) – the speed along the path actually followed
speed made good (SMG) – the speed along the course made good.
standard display
See display category
supplementary information
Non-chart HO information, such as sailing directions, tide tables, light list. (See Fig. 2, 3)
see chart symbol.
System Electronic Navigational Chart (SENC)
A data base resulting from the transformation of the ENC by ECDIS for appropriate use, updates
to the ENC by appropriate means and other data added by the mariner. It is this data base that is
actually accessed by ECDIS for the display generation and other navigational functions, and is

equivalent to an up-to-date paper chart. The SENC may also contain information from other
sources. (See Fig. 1, 2, 3) [PS]
target data
Data on which an update operation is performed by the applier. [TS]
textual HO information
Information presently contained in separate publications (e.g. Sailing Directions) whichmay be
incorporated in the ENC, and also textual information contained in explanatory attributes of
specific objects.
time varying object
An object which has one or more attributes, the value or values of which vary with time.
Note: An example of a time-varying object in S-57 is the geo object “Tide-Time Series” which
gives the times and height of high and low water.
The set of properties of geometic forms (such as connectivity, neighbourhood) which is defined
with the data model remaining invariant when subject to a continuous transformation.
Note: The level of topology chosen for the ENC allows for colour fill, activation of areawarnings,
e.g. depth area warnings, cautionary areas. The different levels of topology are described in the S-
57 Data Model.
The intended path and past path of the ship. When used in conjunction with ECDIS, additional
terminology related to track can include:
planned route (planned track): intended path
past track: past path
cross-track distance: distance right or left of intended path.
track keeping
Sailing a ship in accordance with a pre-determined route, and in relation to the waters. [NAV]
Transfer Standard Maintenance and Application Development Working Group (TSMAD)
A CHRIS working group which replaced the DatabaseWorking Group and the Transfer Standard
Maintenance Working Group and which is responsible for the maintenance of
S-57 and for the production of extensions to S-57 to satisfy new hydrographic requirements.
true distance
Distance on the earth’s surface, based on ellipsoid calculations.
true-motion display
A display in which own ship and each target moves with its own true motion, while the position of
all charted information remains fixed. See also, relative motion display. [ARPA]
A former CHRISWorking Group, now subsumed within the TSMAD WorkingGroup, responsible
for the maintenance of S-57.
The situation where the data displayed are not the largest scale navigational purpose data available
for that area.
Note: An indication should be provided if data of a larger scale navigational purpose is available
for the displayed area.

Either short for update information or, as a verb, applying the update mechanism. See also official
update information
The data which are needed to update the target data automatically. Update information comprises
one or more update records. [TS]
update mechanism
The defined sequence of update operations necessary to update the target data by applying the
update information to the content of the target data so that no operator interaction is involved.
update record
Generic term for feature or spatial records containing update instructions.
update set
The collection of corrections to ENC data promulgated together by an issuing authority on a
schedule basis. [UG]
Note: This is equivalent to the periodic set of Notice toMariners issued for the correction of paper
charts and nautical publications.
Use of the Object Catalogue
Annex A of S-57 Appendix B.1 describing how to encode information relevant to a specific
navigational purpose. Must be used in conjunction with the ENC Product Specification.
Note: Use of the Object Catalogue ismaintained bymeans of new editions, published as required
by the Transfer Standard Maintenance and Applications Development Working Group. The
contents of a new edition do not invalidate the contents of the previous edition.
Direct connection between two points, either given as two sets of coordinates (points), or by
direction and distance from one given set of coordinates, or a point in a vector space defined by
one set of coordinates relative to the origin of a coordinate system. [HD]
vector data presentation
Method of representing individual chart features digitally by points, lines and polygons given
through their coordinates and appropriate code(s). [HD]
voyage data recorder
A system that may be in the form of several separated but interconnected units, intended to
maintain, in a secure and retrievable form, information concerning the position, movement,
physical status, command and control of a vessel over a period leading up to, and following an
incident. Sometimes referred to as Black Box. [VDR]
Note: Not to be confused with the voyage data recording capability of an ECDIS (see voyage
voyage plan
A defined series of waypoints, legs and routes.
voyage recording
ECDIS capability to store and to reproduce certain minimumelements required to reconstruct the
navigation and verify the official data base used during the previous 12 hours. [PS]

An alarm or indicator.
water stabilization
The reference system relative to the water based on course- and speed-through-water sensors.
Note: Used in conjunction with radar/ARPA. See also ground stabilization.
In conjunction with route planning, a geographical location (e.g., latitude and longitude)
indicating a significant event on a vessel’s planned route (e.g., course alteration point, calling in
point, etc.).
See Worldwide Electronic Navigational Chart Data Base.
In computer applications, a rectangular subdivision of the screen showing information without
affecting other parts of the screen; a method allowing to view different items of information (e.g.
different data sets or graphics) at the same time. On graphical screens a window may be selected
with the cursor and subsequently be enlarged (zoom). See also display.
World Geodetic System (WGS)
A global geodetic reference system developed by the USA for satellite position fixing and recommended
by IHO for hydrographic and cartographic use. [HD]
Worldwide Electronic Navigational Chart Data Base (WEND)
A common, worldwide network of ENC datasets, based on IHO standards, designed specifically to
meet the needs of international maritime traffic using ECDIS which conform to the IMO
Performance Standards.
World-Wide NavigationalWarning System (WWNWS)
A service established for the purpose of coordinating the transmission of radio navigational
warnings in geographical areas using coastal and satellite communication services. [HD]
A method of enlarging (zoom in) or reducing (zoom out) graphics displayed on a screen.

Chart Projections

Chart making presents the problem of representing the surface of the Earth, a spheroid, upon a plane surface. The process is one of transferring points on the surface of a sphere or spheroid onto a plane or developable surface, ( a developable surface is one that can be flattened to form a plane).

Desirable Properties in a Chart Projection
– True Shape of Physical Features
– Correct Angular Relationship (Conformal or Orthomorphic)
– Representation of Areas in the Correct Relative Proportions
– Constant Scale Values for Measuring Distance
– Great Circle Represents a Straight Line
– Rhumb Line Represents a Straight Line

Types of Chart Projections
– Cylindrical
– Conic
Simple Conic
Lambert Conformal
– Azimuthal

Sub Types of Chart Projections
– Equatorial
– Polar
– Oblique


Because a cartographer cannot transfer a sphere to a
flat surface without distortion, he must project the surface
of a sphere onto a developable surface. A developable surface
is one that can be flattened to form a plane. This
process is known as chart projection.

Mariners most frequently use a Mercator projection,
classified as a cylindrical projection upon a plane, the cylinder
tangent along the equator. Similarly, a projection
based upon a cylinder tangent along a meridian is called
transverse (or inverse) Mercator or transverse (or inverse)
orthomorphic. The Mercator is the most common
projection used in maritime navigation, primarily because
rhumb lines plot as straight lines.
In a simple conic projection, points on the surface of
the earth are transferred to a tangent cone. In the Lambert
conformal projection, the cone intersects the earth (a secant
cone) at two small circles. In a polyconic projection,
a series of tangent cones is used.

In an azimuthal or zenithal projection, points on the
earth are transferred directly to a plane. If the origin of the
projecting rays is the center of the earth, a gnomonic projection
results; if it is the point opposite the plane’s point of
tangency, a stereographic projection; and if at infinity
(the projecting lines being parallel to each other), an orthographic
projection. The gnomonic, stereographic, and
orthographic are perspective projections. In an azimuthal
equidistant projection, which is not perspective, the scale
of distances is constant along any radial line from the point
of tangency.

304. Cylindrical Projections
If a cylinder is placed around the earth, tangent along
the equator, and the planes of the meridians are extended,
they intersect the cylinder in a number of vertical lines.
These parallel lines of projection are equidistant
from each other, unlike the terrestrial meridians from
which they are derived which converge as the latitude increases.
On the earth, parallels of latitude are perpendicular
to the meridians, forming circles of progressively smaller
diameter as the latitude increases. On the cylinder they are
shown perpendicular to the projected meridians, but because
a cylinder is everywhere of the same diameter, the
projected parallels are all the same size.
If the cylinder is cut along a vertical line (a meridian)
and spread out flat, the meridians appear as equally spaced
vertical lines; and the parallels appear as horizontal lines.
The parallels’ relative spacing differs in the various types of
cylindrical projections.
If the cylinder is tangent along some great circle other
than the equator, the projected pattern of latitude and longitude
lines appears quite different from that described above,
since the line of tangency and the equator no longer coincide.
These projections are classified as oblique or
transverse projections.

Mercator Projection
Navigators most often use the plane conformal projection
known as the Mercator projection. The Mercator projection is
not perspective, and its parallels can be derived mathematically
as well as projected geometrically. Its distinguishing feature is
that both the meridians and parallels are expanded at the same
ratio with increased latitude. The expansion is equal to the secant
of the latitude, with a small correction for the ellipticity of the
earth. Since the secant of 90is infinity, the projection cannot include
the poles. Since the projection is conformal, expansion is
the same in all directions and angles are correctly shown.
Rhumb lines appear as straight lines, the directions of which can
be measured directly on the chart. Distances can also be measured
directly if the spread of latitude is small. Great circles,
except meridians and the equator, appear as curved lines concave
to the equator. Small areas appear in their correct shape but
of increased size unless they are near the equator.

Meridional Parts
At the equator a degree of longitude is approximately
equal in length to a degree of latitude. As the distance from
the equator increases, degrees of latitude remain approximately
the same, while degrees of longitude become progressively shorter.
Since degrees of longitude appear everywhere the same length in
the Mercator projection, it is necessary to increase the length of
the meridians if the expansion
is to be equal in all directions. Thus, to maintain the
correct proportions between degrees of latitude and degrees
of longitude, the degrees of latitude must be progressively
longer as the distance from the equator increases.
The length of a meridian, increased between the equator
and any given latitude, expressed in minutes of arc at the
equator as a unit, constitutes the number of meridional parts
(M) corresponding to that latitude. Meridional parts, given
in Table 6 for every minute of latitude from the equator to
the pole, make it possible to construct a Mercator chart and
to solve problems in Mercator sailing. These values are for
the WGS ellipsoid of 1984.

311. Conic Projections
A conic projection is produced by transferring points
from the surface of the earth to a cone or series of cones.
This cone is then cut along an element and spread out flat to
form the chart. When the axis of the cone coincides with the
axis of the earth, then the parallels appear as arcs of circles,
312. Simple Conic Projection
A conic projection using a single tangent cone is a simple
conic projection (Figure 312a). The height of the cone
increases as the latitude of the tangent parallel decreases. At
the equator, the height reaches infinity and the cone be-
comes a cylinder. At the pole, its height is zero, and the
cone becomes a plane. Similar to the Mercator projection,
the simple conic projection is not perspective since only the
meridians are projected geometrically, each becoming an
element of the cone. When this projection is spread out flat
to form a map, the meridians appear as straight lines converging
at the apex of the cone. The standard parallel,
where the cone is tangent to the earth, appears as the arc of
a circle with its center at the apex of the cone. The other
parallels are concentric circles. The distance along any meridian
between consecutive parallels is in correct relation to
the distance on the earth, and, therefore, can be derived
mathematically. The pole is represented by a circle (Figure
312b). The scale is correct along any meridian and along
the standard parallel. All other parallels are too great in
length, with the error increasing with increased distance
from the standard parallel. Since the scale is not the same in
all directions about every point, the projection is neither a
conformal nor equal-area projection. Its non-conformal nature
is its principal disadvantage for navigation.
Since the scale is correct along the standard parallel
and varies uniformly on each side, with comparatively little
distortion near the standard parallel, this projection is useful
for mapping an area covering a large spread of longitude
and a comparatively narrow band of latitude. It was developed
by Claudius Ptolemy in the second century A.D. to
map just such an area: the Mediterranean Sea.

Lambert Conformal Projection
The useful latitude range of the simple conic projection
can be increased by using a secant cone intersecting the earth
at two standard parallels. See Figure 313. The area between the
two standard parallels is compressed, and that beyond is expanded.
Such a projection is called either a secant conic or
conic projection with two standard parallels.
If in such a projection the spacing of the parallels is altered,
such that the distortion is the same along them as
along the meridians, the projection becomes conformal.
This modification produces the Lambert conformal projection.
If the chart is not carried far beyond the standard
parallels, and if these are not a great distance apart, the distortion
over the entire chart is small.
A straight line on this projection so nearly approximates a
great circle that the two are nearly identical. Radio beacon signals
travel great circles; thus, they can be plotted on this
projection without correction. This feature, gained without sacrificing
conformality, has made this projection popular for
aeronautical charts because aircraft make wide use of radio aids
to navigation. Except in high latitudes, where a slightly modified
form of this projection has been used for polar charts, it has not
replaced the Mercator projection for marine navigation.

Polyconic Projection
The latitude limitations of the secant conic projection can
be minimized by using a series of cones. This results in a Polyconic
projection with two standard parallels.
In this projection, each parallel is the base of
a tangent cone. At the edges of the chart, the area between parallels
is expanded to eliminate gaps. The scale is correct along
any parallel and along the central meridian of the projection.
Along other meridians the scale increases with increased difference
of longitude from the central meridian. Parallels appear as
nonconcentric circles; meridians appear as curved lines converging
toward the pole and concave to the central meridian.
The polyconic projection is widely used in atlases, particularly
for areas of large range in latitude and reasonably
large range in longitude, such as continents. However, since
it is not conformal, this projection is not customarily used
in navigation.

316. Gnomonic Projection
If a plane is tangent to the earth, and points are projected
geometrically from the center of the earth, the result is a
gnomonic projection. See Figure 316a. Since the projection
is perspective, it can be demonstrated by placing a light
at the center of a transparent terrestrial globe and holding
a flat surface tangent to the sphere.
In an oblique gnomonic projection the meridians appear
as straight lines converging toward the nearer pole. The
parallels, except the equator, appear as curves (Figure
316b). As in all azimuthal projections, bearings from the
point of tangency are correctly represented. The distance
scale, however, changes rapidly. The projection is neither
conformal nor equal area. Distortion is so great that shapes,
as well as distances and areas, are very poorly represented,
except near the point of tangency.
michael 08.4822/N