units.xsd v3.0 draft last updated 2002/04/22 GML 3.0 candidate schema. Copyright (c) 2002 OGC, All Rights Reserved. This schema encodes the Coordinate Reference Systems (SC_) package of the CRS UML Model, with the exception of the abstract CRS class. A coordinate reference system consists of a set of coordinate system axes that is related to the earth through a datum. A coordinate reference system is realised by a set of coordinates. The realisation is sometimes known as a reference frame. A coordinate reference system shall be defined by one datum and by one coordinate system. For the purposes of this Abstract Specification a coordinate reference system shall not change with time with the exception of engineering coordinate reference systems defined on moving platforms such as cars, ships, aircraft, and spacecraft. The intention is to exclude the option to describe the time variability of geodetic coordinate reference systems as a result of e.g. tectonic motion. This variability is part of the subject matter of geophysical and geodetic science. The model for spatial referencing by coordinates described in this Abstract Specification is in principle not suitable for such zero-order geodetic problems. Such time-variability of coordinate reference systems shall be covered in the spatial referencing model described in this document by creating different coordinate reference systems each with a different datum for (consecutive) epochs. The date of realisation of the datum shall be included in its definition. It is further recommended to include the date of realisation also in the names of the datums and the coordinate reference systems. Coordinate reference systems are commonly divided into sub-types. The common classification criterion for sub-typing of coordinate reference systems is the way in which they deal with earth curvature. This has a direct effect on the portion of the earth's surface that can be covered by that type of CRS with an acceptable degree of error. The exception to the rule is the subtype "Temporal" which has been added by analogy. A coordinate reference system describing the position of points through two or more independent coordinate reference systems. To be supplied. Association to a Compound Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. Association to a Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. A coordinate reference system that deals with the earth's curvature by taking the 3D spatial view which obviates the need to model the earth's curvature. The origin of a geocentric CRS is at the approximate centre of mass of the earth. To be supplied. To be supplied. Association to a Geocentric Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. To be supplied. A coordinate reference system based on an ellipsoidal approximation of the geoid; this provides an accurate representation of the geometry of geographic features for a large portion of the earth's surface. To be supplied. To be supplied. Association to a Geographic Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. To be supplied. To be supplied. To be supplied. Association to a Derived Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. To be supplied. A coordinate reference system used to approximate the shape of the earth on a planar surface, but in such a way that the distortion that is inherent to the approximation is carefully controlled and known. Distortion correction is commonly applied to calculated bearings and distances to produce values that are a close match to actual field values. To be supplied. To be supplied. Association to a Projected Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. A contextually local coordinate reference system; which can be divided into two broad categories: - earth-fixed systems applied to engineering activities on or near the surface of the earth; - CRSs on moving platforms such as road vehicles vessels aircraft or spacecraft. To be supplied. To be supplied. Association to an Engineering Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. An engineering coordinate reference systems applied to images. Image spatial referencing systems are treated as a separate sub-type because a separate user community exists for images with its own terms of reference. To be supplied. To be supplied. Association to an Image Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. To be supplied. To be supplied. To be supplied. To be supplied. Association to a Vertical Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System. To be supplied. To be supplied. To be supplied. Association to a Temporal Coordinate Reference System, either referencing or containing the definition of that Coordinate Reference System.