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Coordinate reference systems

Definition:

Systems for uniquely referencing spatial information in space as a set of coordinates (x,y,z) and/or latitude and longitude and height, based on a geodetic horizontal and vertical datum.

Description:

The theme establishes a structure for spatial referencing of features by coordinates. This topic shall link to appropriate standards for information technology and data where possible, and provide a framework for the development of sector-specific applications using geographic data.

ISO 19111 describes the conceptual schema and defines the description for a minimum data to two cases for which 1-, 2- and 3- dimensional coordinates reference system information shall be given. The first case is given by a coordinate reference system to which a set of coordinates is related. The second case consists of a coordinate operation (coordinate transformation, coordinate conversion, concatenated coordinate operation) to change coordinate values from one coordinate reference system to another.

There are no explicit accuracy numbers given in ISO 19111. We must consider that it has been developed for geographic information in general, but not for precise positioning. Spatial information may be referenced to the earth surface with an improving accuracy on the global scale for the future. Such high accuracy level may be required for some themes of the Directive, e.g., the trans-European railway transport network. Spatial referencing could no longer be considered as constant in time, if we reach the sub-centimetre level. We need additional parameters compared to ISO 19111 in that case, because that document considers changes in time of the coordinate reference only system through the “date of realisation”. This model is not suitable to describe continuous movements of the spatial reference. Kinematic models or so-called “loading models” are examples to incorporate such dynamics. A re-iteration of accuracy aspects may be needed, if specifications for Annex I and II are ready.

The spatial referencing is usually referred to selected points of the earth surface. Such point are, e.g., given by geodetic markers, stations performing permanent satellite observations, levelling benchmarks, or tide gauges. As soon as the marker coordinates are given, they provide a direct access to the realisation of the coordinate reference system.


Scope, use examples:

All users of GI-data need geodetic reference data to be in place. From that point of view the coordinate reference systems are a prerequisite for a successful realisation of all themes of the Directive. The use of GNSS for accurate mapping needs special services that provide various information and corrections from GNSS permanent reference stations (accuracy level 1 m to 1 cm). Selected themes, e.g., sea level rise, require the spatial reference and the corresponding changes of control stations for better than 1 mm/year.

ISO 19111 could be used as a basis for the implementation of coordinate reference systems in a European infrastructure for spatial information. The ISO document describes the definition of a coordinate system as well as coordinate operations to change from one coordinate system to another one. Changes in time are only considered in ISO 19111 through the date of realisation. It has to be studied, whether kinematic spatial information could be described by the attributes of coordinate operations as determined in ISO 19111. Alternatively new attributes will have to be defined in addition to the ISO standard to implement this theme. Implementation rules for coordinate reference systems should account for reference systems that are realised in real-time, e.g., through a GNSS real-time correction service provider. It has to be confirmed that ISO 19111 features are suitable to describe the reference system information as transmitted by the service provider. The full set of reference information data will not likely be transmitted by the correction service and thus requires further conventions.

The ETRS89 is an example for a coordinate reference system in Europe, which has been adopted by the European Commission (ref COGI action decision 2003 - F/GIS/69/EN). It is today realised through a network of more than 200 permanent operating GNSS observing stations of the EUREF organization. This realisation not only provides static, but furthermore kinematic information of spatial referencing. This geodetic reference is widely used in continental Europe. Furthermore the increasing use of GPS networks incline countries to use the European system.

The new European Satellite Navigation System GALILEO will maintain its own coordinate reference frame, the Galileo Terrestrial Reference Frame (GTRF). It will be aligned to the International Terrestrial Reference Frame (ITRF) and is covered by the ISO 19111 standard.

The European Vertical Reference System (EVRS) is a gravity related height system and was defined by EUREF. Gravity related heights are required to describe various environmental phenomena, e.g., all occurrences concerning water level. EVRS was recommended the European Commission as reference height system for geo data. It is realised by the geopotential numbers and normal heights of nodal points of the United European Levelling Network 95/98 (UELN 95/98) to which the name EVRF 2000 is given. Significant improvements to the realisation of EVRS are expected in the coming years from the ESA satellite mission GOCE. Similarly, developments in GNSS like GALILEO will improve the ETRS89 realizations. An appropriate mechanism to accommodate new, improved coordinates in the INSPIRE is the version numbering according to ISO 19108. This issue is independent of the kinematic aspects discussed previously.

ETRS89 and EVRS could be implemented in a spatial information system following the ISO 19111 standards with the above mentioned restrictions concerning kinematic aspects.

Many national reference frames are distorted. Therefore, coordinates can only be transformed to the ETRS89 system with decimeter to meter accuracies.

An example of Coordinate operations in the scope of ISO 19111 is realised through the “Information and Service for European Coordinate Reference Systems (CRS)” at http://crs.bkg.bund.de, which was established by BKG, EuroGeographics and EUREF. This system provides among others the mathematical parameters to change the coordinates from national reference systems in Europe to the ETRS89.

Today, most Web services identify coordinate reference systems by means of the EPSG codes (see www.epsg.org).

Important feature types and attributes:

The following important features are extracted from ISO 19111 for

a) Coordinate reference system

  • Coordinate reference system identifier;
  • Datum identifier;
  • Datum type;
  • Datum anchor point;
  • Datum realization epoch;
  • Datum valid area;
  • Ellipsoid identifier;
  • Ellipsoid flattening.

and

b) Coordinate operation

  • Coordinate operation identifier;
  • Coordinate operation valid area;
  • Source coordinate reference system identifier;
  • Target coordinate reference system identifier;
  • Coordinate operation method name;
  • Coordinate operation method formula;
  • Coordinate operation parameter value.


Links and overlaps with other themes:

The coordinate reference systems are a prerequisite for a successful realisation of all themes of the Directive.

Reference documents:

International Standard ISO 19111:2003, Geographic information – Spatial referencing by coordinates

EUREF Publication No. 14 - Report on the Symposium of the IAG Sub-commission for Europe (EUREF) held in Bratislava, 2 – 5 June 2004

Map Projections for Europe. Institute for Environment and Sustainability, JRC, EC, 2003. COGI action decision 2003 - F/GIS/69/EN

From the reference material submitted by SDICs and LMOs, many documents relate with this theme.
See categorised list of reference material, compiled by JRC

 

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Last updated 2930 days ago by INSPIRE Forum