Reference numberSecond edition2008-07-15 Standard representation of geographic point location by coordinates Représentation normalisée des latitude, longitude et altitude pour la locali
Conceptual model for geographic point locations
A coordinate is one of a sequence of numbers describing the position of a point A coordinate tuple is composed of a sequence of coordinates describing one position
EXAMPLE A coordinate tuple consisting of latitude, longitude and height represents a 3-dimensional geographic position
A coordinate tuple can only precisely represent a location if its associated coordinate reference system (CRS) is identified, ensuring unambiguous positioning Without specifying the CRS, there could be significant uncertainty, potentially resulting in a positional error of several hundred metres ISO 19111 outlines the essential elements needed to accurately describe a coordinate reference system, facilitating precise and reliable location referencing.
A coordinate set consists of multiple coordinate tuples that are all referenced to the same coordinate reference system, as mandated by ISO 19111 When describing a single point, the link between the coordinate tuple and CRS can be direct; however, for a set, one CRS identification applies to all tuples within the set The conceptual relationship between coordinate tuples, coordinate sets, and coordinate reference systems is visually represented in Figure 1 and formally detailed in UML in Annex C Ensuring all coordinate tuples in a set share the same CRS is essential for spatial data consistency and accurate georeferencing.
Figure 1 — Conceptual relationship of coordinates to a Coordinate Reference System (CRS)
Coordinates within a 2-dimensional coordinate reference system (CRS) specify a geographic location horizontally To accommodate the vertical dimension essential in modern geospatial applications, the International Standard now supports 3-dimensional coordinate representations A 3D geographical point can be described using either a dedicated 3D CRS or a compound CRS that combines separate horizontal and vertical CRSs A coordinate reference system consists of a coordinate system and a datum, providing a standardized framework for accurately representing spatial positions.
Figure 2 — Conceptual model of a Coordinate Reference System
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Elements required for geographic point location
In this International Standard, geographic point location shall be represented by four elements:
⎯ coordinate representing “x” horizontal position such as latitude;
⎯ coordinate representing “y” horizontal position such as longitude;
⎯ for 3-dimensional point locations, a value representing vertical position through either height or depth;
Coordinate Reference System identification
A CRS (Coordinate Reference System) identification is essential for unambiguously describing geographic point locations, including their vertical positions A compound CRS identification should be provided for points with both horizontal and vertical coordinates to ensure precise positioning Without an appropriate CRS, there is an inherent uncertainty in the geographic location, which can result in positional offsets of up to 1 km from the actual point, as detailed in Annex B Proper CRS specification is crucial for accurate geographic data interpretation and minimizing positional errors.
A Coordinate Reference System (CRS) description should be provided either by referencing a definition in a register of geodetic codes that complies with ISO/TS 19127 standards or by offering a comprehensive CRS definition as specified in ISO 19111 This approach ensures clarity and consistency in geospatial referencing, adhering to international standards for accuracy and interoperability Proper documentation of CRS details is essential for accurate spatial data management and GIS applications.
Methods a) and b) offer alternative approaches to providing a complete CRS (Coordinate Reference System) definition Method a) is recommended for its simplicity, while Method b) should be used when the system definition is not available from a register, requiring a full specification In both methods, the CRS explicitly defines the order of coordinates within each coordinate tuple, along with the units of measurement and the representation of the coordinate values, ensuring precise and consistent spatial referencing.
For certain interchange purposes, confirming the identity of a coordinate reference system (CRS) using a geodetic register is sufficient, without requiring the full system definition Method a) allows applications that only need to verify CRS identification to reference the register citation and CRS unique identifier, avoiding retrieval of the complete CRS components unless necessary for quoting or coordinate operations The syntax for CRS definition via a geodetic register, as outlined in method a), is designed to streamline CRS identification and facilitate efficient data exchange.
1) for an online register: crsName=”url”
EXAMPLE crsName=”http://www.xxxx.org#xxxx:1234”; or
2) for a register which is not online: crsName=[registerID]:[register’s CRS ID]
Representation of horizontal position
Horizontal positioning is defined using a pair of coordinates according to any coordinate reference system (CRS) specified in ISO 19111, ensuring clarity in positive axes directions, coordinate order, and units as outlined in the CRS definition If no CRS is provided, default assumptions shall be applied to maintain consistent and accurate location referencing.
According to ISO 2008 standards, latitude values should precede longitude values within coordinate tuples, with latitudes north of or on the equator being positive and those south of the equator negative Longitudes east of or on the prime meridian are positive, while those west are negative, with the 180th meridian designated as negative, and the prime meridian established at Greenwich For digital data exchange, decimal degrees are the preferred format, though sexagesimal degrees may still be used for backward compatibility Recommendations for displaying latitude and longitude at the human interface are provided in Annex D.
Representation of vertical position
Vertical position refers to height or depth as defined by the coordinate reference system Heights measured upward from the origin are positive, while those measured downward are negative Conversely, depths measured downward from the origin are positive, and depths measured upward are negative, ensuring consistent representation of vertical spatial data.
When specifying height or depth, the CRS definition must clearly identify whether the value represents height or depth, specify its position within the coordinate tuple, include the relevant unit of measurement, and define the origin point utilized for these measurements.
Coordinate resolution
Coordinates must be provided with a resolution that matches the required position accuracy, which can be documented through metadata as specified in ISO 19115 The linear equivalents for angular coordinates, such as latitude and longitude, are detailed in Annex E, ensuring precise and standardized geospatial referencing.
Utilization of geographic point locations
ISO 19115 specifies the standardized requirements for describing metadata related to geospatial information, ensuring consistent and comprehensive documentation It includes detailed examples, such as geographic point locations with their coordinates and associated attributes like date stamps and descriptive information, which are outlined in Annex F Adhering to ISO 19115 enhances data interoperability, discoverability, and management of geospatial datasets.
7 Representation of geographic point location
UML model
The UML model for the representation of geographic point location is described in Annex C.
XML representation
This International Standard supports GML, an XML grammar developed within an XML schema designed for describing application schemas, as well as for the transport and storage of geographic information It offers flexibility in representing geographic point locations, which will be managed through a dedicated register Examples of geographic point location representations using GML are provided in Annex G.
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Text string representation
This edition extends the representation of geographic point locations—previously defined by latitude, longitude, and optional height or depth—to include Coordinate Reference System (CRS) identification When using this optional text string to exchange geographic point location information, it must conform to the standardized format specified in Annex H, ensuring clarity and consistency across implementations.
Conformance and abstract test suite
A.1 Conformance with this International Standard
Conformance requirement: Any geographic point location, expressed by coordinates, shall pass all the requirements described in the following abstract test suite
A.1.2 Abstract test suite for conformance
This test case (A.1.2.1) verifies that all essential elements for a geographic point location are complete and correctly present The primary purpose is to ensure the completeness of required location elements, following the guidelines outlined in section 6.2 The test involves checking and confirming that all specified elements are included, making it a fundamental validation process to maintain accurate geographic data integrity.
The test case aims to verify that the CRS description retrieved from the register is complete and unambiguous, ensuring it provides a clear and sufficient definition applicable to the specific location The testing method involves reviewing the description to confirm its clarity and applicability, referencing sections 6.3 and ISO/TS 19127 This is classified as a basic test type, essential for validating the integrity of coordinate reference system information.
The test case for CRS definition aims to verify that the complete Coordinate Reference System (CRS) aligns with ISO 19111 standards This involves assessing whether the CRS definition contains all required information in the correct format and sequence as specified by ISO 19111, following the guidelines outlined in section 6.3 The testing method includes checking the provided definition against ISO 19111 requirements, ensuring all necessary details are accurately represented This is a fundamental, basic test type designed to confirm compliance with international geospatial standards.
The test case, identified as A.1.2.4, aims to verify that the representation of the horizontal position complies with the elements outlined in section 6.4 This involves checking all specified requirements to ensure that the necessary information is accurately provided in the designated format and sequence, ensuring adherence to the standard specifications.
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This test case, identified as A.1.2.5, aims to verify that the representation of the horizontal position aligns with the specifications outlined in section 6.5 The testing method involves reviewing the requirement details in section 6.5 to ensure all necessary information is provided correctly in the specified format and sequence This is a basic test designed to confirm compliance with the established standards for representing the horizontal position.
The test case with the identifier "text string representation" aims to verify that the representation complies with the relevant International Standard, ensuring accuracy and consistency The test involves examining the requirements specified in Annex H to confirm that all necessary information is correctly formatted and sequenced This is a basic test type, referencing section 7.3, designed to validate that the text string accurately reflects the standard's specifications.
Latitude and longitude coordinates are not unique
Latitude and longitude are key measurements used to reference locations on Earth's surface, typically based on an ellipsoid model Several hundred models have been developed, with around forty still actively in use for daily applications The choice of a geodetic model, along with its position and orientation relative to Earth, defines the geodetic datum, which is essential for accurate positioning Changing the model or the datum’s position or orientation alters the latitude and longitude values at a given point Therefore, referencing the same coordinates to different datums results in different geographical locations, emphasizing the importance of clearly identifying the datum to ensure unambiguous location referencing.
NOTE “WGS 84”, “ED50” and “OSGB 1936” are the identifiers of Coordinate Reference Systems
Figure B.1 — Locations with identical latitude and longitude values on three different datums
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Projected coordinates are derived from geographic (latitude and longitude) coordinates For the projected coordinates to be unambiguous, the datum for their source geographic coordinates must therefore also be identified
The differences in coordinate values of a point caused by a change of geodetic datum are typically about 50 to
When working with coordinate accuracy around 1 km or worse, differences of up to 500 meters or more are generally not significant However, for applications requiring greater precision—better than approximately 1 km—it is vital to identify the correct datum to ensure unambiguous coordinates Accurate datum identification is essential for maintaining the integrity and reliability of spatial data in high-precision applications.
UML description for representation of geographic point locations
This annex provides a formal description of coordinate tuples and coordinate sets, which are essential geometric constructs referenced in this International Standard These constructs are interconnected with other ISO geographic information standards, as illustrated in Figure C.1 Detailed representations of these geometric constructs are provided in UML notation in Figure C.2, in accordance with ISO/TS 19103, ensuring clarity and compliance with global geospatial data standards.
This UML description is designed to facilitate the accurate representation of geographic point locations, supporting various methods of visualization and data integration Key classes within the diagram provide essential functionality for transforming geographic coordinates and identifying different coordinate representations, ensuring flexibility and precision in geographic data management Implementing these features enhances the ability to effectively model and utilize geographic point data across diverse applications.
The dependencies for the UML class diagram are shown in Figure C.2 Each dependency contains classes that are reused in this International Standard These are as follows:
⎯ ISO/TS 19103: Conceptual schema language;
⎯ ISO 19111: Spatial referencing by coordinates;
⎯ ISO 19133: Location based services — Tracking and navigation
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C.3 Description of the UML GPL package components
This subclause describes in detail the UML class diagram presented in Figure C.3 This class diagram has three classes to support multiple coordinate representations: GPL_CoordinateRepresentation, GPL_CoordinateTuple, GPL_CoordinateSet
The GPL_CoordinateRepresentation is a union data type that can store either a CharacterString for text representation or BinaryData for binary number representation This flexible structure supports various formats, including multiple types of XML representations, making it suitable for diverse data encoding needs By providing both textual and binary options, it ensures efficient and compatible data handling for coordinate information.
A GPL_CoordinateTuple is a specialized data type designed to represent geographic point locations accurately It contains an attribute that stores the coordinates of a specific location and includes operations to perform essential transformations for coordinate representation This makes GPL_CoordinateTuple essential for precise geospatial data handling and mapping applications.
A tuple represents the coordinates of a position within a DirectPosition data type, providing precise spatial information According to ISO 19107, the DirectPosition enables the association of a coordinate reference system with this point, ensuring accurate geographic positioning Understanding tuples and DirectPosition is essential for geospatial referencing and spatial data accuracy.
The operation “encode” performs the representation of this GPL_CoordinateTuple in a GPL_CoordinateRepresentation
GPL_CoordinateTuple::encode(repType : CharacterString) :
The "decode" operation converts a GPL_CoordinateRepresentation into a GPL_CoordinateTuple, ensuring seamless data interpretation If multiple representations of a point are included within the GPL_CoordinateRepresentation, the operation will decode only the first one This process simplifies coordinate data processing by focusing on the primary representation Using "decode" enhances data accuracy and consistency in spatial data workflows.
GPL_CoordinateTuple::decode(repType : CharacterString, repCoordTuple :
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The operation “GM_Point” returns the GPL_CoordinateTuple as a GM_Point data type in conformance with ISO 19107
GPL_CoordinateTuple::GM_Point() : GM_Point
The operation “GM_Position” returns the GPL_CoordinateTuple as a GM_Position data type in conformance with ISO 19107
GPL_CoordinateTuple::GM_Position() : GM_Position
The operation “asSequence” returns the GPL_CoordinateTuple as a sequence of numbers in conformance with ISO/TS 19103
GPL_CoordinateTuple::asSequence() : Sequence
“metadata” is an association role to reference metadata elements as described in ISO 19115 to a GPL_CoordinateTuple
GPL_CoordinateTuple::metadata [0,1] : ISO19115::MD_Metadata
A GPL_CoordinateSet is a data type that aggregates a number of GPL_CoordinateTuples Similarly to the
“CoordinateTuple”, CoordinateSet has operations to perform the necessary transformations for the coordinate representation of one or multiple points
“coordTuple” is an association role that aggregates references of a number of GPL_CoordinateTuples
GPL_CoordinateSet::coordTuple [1,n] : Reference
The operation “encode” performs the representation of this GPL_CoordinateSet in a GPL_CoordinateRepresentation
GPL_CoordinateSet::encode(repType : CharacterString) :