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/*
* GeoTools - The Open Source Java GIS Toolkit
* http://geotools.org
*
* (C) 2011, Open Source Geospatial Foundation (OSGeo)
* (C) 2004-2005, Open Geospatial Consortium Inc.
*
* All Rights Reserved. http://www.opengis.org/legal/
*/
/**
* {@linkplain org.opengis.referencing.crs.CoordinateReferenceSystem Coordinate reference systems}
* ({@linkplain org.opengis.referencing.cs.CoordinateSystem coordinate systems} with a {@linkplain
* org.opengis.referencing.datum.Datum datum}). The following is adapted from OpenGIS® Spatial Referencing
* by Coordinates (Topic 2) specification.
*
* A coordinate reference system consists of one coordinate system that is
* related to the earth through one datum. The coordinate system is composed of a set of coordinate
* axes with specified units of measure. This concept implies the mathematical rules that define how
* coordinate values are calculated from distances, angles and other geometric elements and vice
* versa.
*
*
A datum specifies the relationship of a coordinate system to the earth, thus
* ensuring that the abstract mathematical concept "coordinate system" can be applied to the
* practical problem of describing positions of features on or near the earth's surface by means of
* coordinates. The resulting combination of coordinate system and datum is a coordinate reference
* system. Each datum subtype can be associated with only specific types of coordinate systems. The
* datum implicitly (occasionally explicitly) contains the values chosen for the set parameters that
* represent the degrees of freedom of the coordinate system. A datum therefore implies a choice
* regarding the approximate origin and orientation of the coordinate system.
*
*
For the purposes of this 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 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
* specification by creating different coordinate reference systems, each with a different datum,
* for (consecutive) epochs. The date of realisation of the datum shall then be included in its
* definition. It is further recommended to include the date of realisation in the names of those
* datums and coordinate reference systems.
*
*
*
*
Principal sub-types of coordinate reference system
*
* Geodetic survey practice usually divides coordinate reference systems into a
* number of sub-types. The common classification criterion for sub-typing of coordinate reference
* systems can be described as 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. Thus the following principal sub-types of coordinate reference system
* are distinguished:
*
*
*
* Geocentric: Type of 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.
*
*
Geographic: Type of 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. Geographic coordinate reference
* systems can be 2D or 3D. A 2D Geographic CRS is used when positions of features are described on
* the surface of the reference ellipsoid; a 3D Geographic CRS is used when positions are described
* on, above or below the reference ellipsoid.
*
*
Projected: Type of coordinate reference system that is based on an
* approximation of the shape of the earth's surface by a plane. 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.
*
*
Engineering: Type of coordinate reference system that is that is used
* only in a contextually local sense. This sub-type is used to model two broad categories of local
* coordinate reference systems:
*
*
* - earth-fixed systems, applied to engineering activities on or near the surface of the earth;
*
- coordinates on moving platforms such as road vehicles, vessels, aircraft or spacecraft.
*
*
* Image: An Image CRS is an Engineering CRS applied to images. Image CRSs
* are treated as a separate sub-type because a separate user community exists for images with its
* own vocabulary. The definition of the associated Image Datum contains two data attributes not
* relevant for other datums and coordinate reference systems.
*
*
Vertical: Type of coordinate reference system used for the recording of
* heights or depths. Vertical CRSs make use of the direction of gravity to define the concept of
* height or depth, but its relationship with gravity may not be straightforward. By implication
* ellipsoidal heights (h) cannot be captured in a vertical coordinate reference system. Ellipsoidal
* heights cannot exist independently, but only as inseparable part of a 3D coordinate tuple defined
* in a geographic 3D coordinate reference system.
*
*
Temporal: Used for the recording of time in association with any of the
* listed spatial coordinate reference systems only.
*
*
*
*
*
*
Additional sub-types of coordinate reference system
*
* In addition to the principal sub-types, so called because they represent
* concepts generally known in geodetic practice, two more sub-types have been defined to permit
* modelling of certain relationships and constraints that exist between the principal sub-types.
* These additional sub-types are Compound coordinate reference system
* and Derived coordinate reference system.
*
*
*
* Compound coordinate reference system
* The traditional separation of horizontal and vertical position has resulted in coordinate
* reference systems that are horizontal (2D) in nature and vertical (1D). It is established
* practice to combine the horizontal coordinates of a point with a height or depth from a different
* coordinate reference system. The coordinate reference system to which these 3D coordinates are
* referenced combines the separate horizontal and vertical coordinate reference systems of the
* horizontal and vertical coordinates. Such a coordinate system is called a compound coordinate
* reference system (Compound CRS). It consists of an ordered sequence of the two or more single
* coordinate reference systems.
*
*
Derived coordinate reference system
* Some coordinate reference systems are defined by applying a coordinate conversion to another
* coordinate reference system. Such a coordinate reference system is called a Derived CRS and the
* coordinate reference system it was derived from by applying the conversion is called the Source
* or Base CRS. A coordinate conversion is an arithmetic operation with zero or more parameters that
* have defined values. The Source CRS and Derived CRS have the same Datum. The best-known example
* of a Derived CRS is a Projected CRS, which is always derived from a source Geographic CRS by
* applying the coordinate conversion known as a map projection.
*
*
In principle, all sub-types of coordinate reference system may take on the
* role of either Source or Derived CRS with the exception of a Geocentric CRS and a Projected CRS.
* The latter is modelled as an object class under its own name, rather than as a general Derived
* CRS of type "projected". This has been done to honour common practice, which acknowledges
* Projected CRSs as one of the best known types of coordinate reference systems.
*
*
An example of a Derived CRS: one of which the unit of measure has been
* modified with respect to an earlier defined Geographic CRS, which then takes the role of Source
* CRS.
*
*
*
* @version Abstract
* specification 2.0
* @since GeoAPI 1.0
*/
package org.opengis.referencing.crs;