/*
* 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.datum.Datum Geodetic datum} (the relationship of a
* {@linkplain org.opengis.referencing.cs.CoordinateSystem coordinate system} to the earth).
* The following is adapted from
* <A HREF="http://portal.opengeospatial.org/files/?artifact_id=6716">OpenGIS®
* Spatial Referencing by Coordinates (Topic 2)</A> specification.
*
* <P ALIGN="justify">A datum specifies the relationship of a coordinate system
* to the earth or, in some applications to an Engineering CRS, to a moving
* platform, thus creating a coordinate reference system. A datum can be used
* as the basis for one-, two- or three-dimensional systems.</P>
*
* <P ALIGN="justify">Five subtypes of datum are specified: geodetic, vertical,
* engineering, image and temporal. Each datum subtype can be associated only
* with specific types of coordinate reference systems. A geodetic datum is used
* with three-dimensional or horizontal (two-dimensional) coordinate reference
* systems, and requires an ellipsoid definition and a prime meridian definition.
* It is used to describe large portions of the earth's surface up to the entire
* earth's surface. A vertical datum can only be associated with a vertical
* coordinate reference system. Image datum and engineering datum are both used
* in a local context only: to describe the origin of an image and the origin of
* an engineering (or local) coordinate reference system.</P>
*
* <P> </P>
* <H3>Vertical datum</H3>
* <P ALIGN="justify">Further sub-typing is required to describe vertical datums
* adequately. The following types of vertical datum are distinguished:</P>
* <UL>
* <LI><P ALIGN="justify"><STRONG>Geoidal</STRONG><BR>
* The zero value of the associated (vertical) coordinate system axis is defined
* to approximate a constant potential surface, usually the geoid. Such a reference
* surface is usually determined by a national or scientific authority and is then
* a wellknown, named datum. This is the default vertical datum type, because it is
* the most common one encountered.</P></LI>
*
* <LI><P ALIGN="justify"><STRONG>Depth</STRONG><BR>
* The zero point of the vertical axis is defined by a surface that has meaning for
* the purpose the associated vertical measurements are used for. For hydrographic
* charts, this is often a predicted nominal sea surface (i.e., without waves or
* other wind and current effects) that occurs at low tide. Examples are Lowest
* Astronomical Tide and Lowest Low Water Spring. A different example is a sloping
* and undulating River Datum defined as the nominal river water surface occurring
* at a quantified river discharge.</P></LI>
*
* <LI><P ALIGN="justify"><STRONG>Barometric</STRONG><BR>
* A vertical datum is of type "barometric" if atmospheric pressure is the basis
* for the definition of the origin. Atmospheric pressure may be used as the
* intermediary to determine height (barometric height determination) or it may
* be used directly as the vertical ordinate, against which other parameters are
* measured. The latter case is applied routinely in meteorology.</P>
*
* <P ALIGN="justify">Barometric height determination is routinely used in aircraft.
* The altimeter (barometer) on board is set to the altitude of the airfield at the
* time of take-off, which corrects simultaneously for instantaneous air pressure and
* altitude of the airfield. The measured height value is commonly named "altitude".</P>
*
* <P ALIGN="justify">In some land surveying applications height differences between
* points are measured with barometers. To obtain absolute heights the measured height
* differences are added to the known heights of control points. In that case the vertical
* datum type is not barometric, but is the same as that of the vertical control network
* used to obtain the heights of the new points and its vertical datum type.</P>
*
* <P ALIGN="justify">The accuracy of this technique is limited, as it is affected
* strongly by the spatial and temporal variability of atmospheric pressure. This
* accuracy limitation impacts the precision of the associated vertical datum definition.
* The datum is usually the surface of constant atmospheric pressure approximately
* equating to mean sea level (MSL). The origin or anchor point is usually a point
* of known MSL height. The instruments are calibrated at this point by correcting
* for the instantaneous atmospheric pressure at sea level and the height of the
* point above MSL.</P>
*
* <P ALIGN="justify">In meteorology, atmospheric pressure routinely takes the role
* as vertical ordinate in a CRS that is used as a spatial reference frame for
* meteorological parameters in the upper atmosphere. The origin of the datum
* is in that case the (hypothetical) zero atmospheric pressure and the positive
* vertical axis points down (to increasing pressure).</P></LI>
*
* <LI><P ALIGN="justify"><STRONG>Other surface</STRONG><BR>
* In some cases, e.g. oil exploration and production, geological features,
* i.e., the top or bottom of a geologically identifiable and meaningful subsurface
* layer, are sometimes used as a vertical datum. Other variations to the above three
* vertical datum types may exist and are all bracketed in this category.</P></LI>
* </UL>
*
* <P> </P>
* <H3>Image datum</H3>
* <P ALIGN="justify">The image pixel grid is defined as the set of lines of constant
* integer ordinate values. The term "image grid" is often used in other standards to
* describe the concept of Image CRS. However, care must be taken to correctly interpret
* this term in the context in which it is used. The term "grid cell" is often used as a
* substitute for the term "pixel".</P>
*
* <P ALIGN="justify">The grid lines of the image may be associated in two ways with
* the data attributes of the pixel or grid cell (ISO CD 19123). The data attributes
* of the image usually represent an average or integrated value that is associated
* with the entire pixel.</P>
*
* <P ALIGN="justify">An image grid can be associated with this data in such a way
* that the grid lines run through the centres of the pixels. The cell centres will
* thus have integer coordinate values. In that case the attribute "pixel in cell"
* will have the value "cell centre".</P>
*
* <P ALIGN="justify">Alternatively the image grid may be defined such that the
* grid lines associate with the cell or pixel corners rather than the cell centres.
* The cell centres will thus have noninteger coordinate values, the fractional parts
* always being 0.5. ISO CD 19123 calls the grid points in this latter case "posts"
* and associated image data: "matrix data". The attribute "pixel in cell" will now
* have the value "cell corner".</P>
*
* <P ALIGN="justify">This difference in perspective has no effect on the image
* interpretation, but is important for coordinate transformations involving this
* defined image.</P>
*
* <P> </P>
* <H3>Prime meridian</H3>
* <P ALIGN="justify">A prime meridian defines the origin from which longitude values
* are specified. Most geodetic datums use Greenwich as their prime meridian. A prime
* meridian description is not needed for any datum type other than geodetic, or if the
* datum type is geodetic and the prime meridian is Greenwich. The prime meridian
* description is mandatory if the datum type is geodetic and its prime meridian
* is not Greenwich.</P>
*
* <P> </P>
* <H3>Ellipsoid</H3>
* <P ALIGN="justify">An ellipsoid is defined that approximates the surface of the
* geoid. Because of the area for which the approximation is valid - traditionally
* regionally, but with the advent of satellite positioning often globally - the
* ellipsoid is typically associated with Geographic and Projected CRSs. An ellipsoid
* specification shall not be provided if the datum type not geodetic.</P>
*
* <P ALIGN="justify">One ellipsoid must be specified with every geodetic datum,
* even if the ellipsoid is not used computationally. The latter may be the case
* when a Geocentric CRS is used, e.g., in the calculation of satellite orbit and
* ground positions from satellite observations. Although use of a Geocentric CRS
* apparently obviates the need of an ellipsoid, the ellipsoid usually played a role
* in the determination of the associated geodetic datum. Furthermore one or more
* Geographic CRSs may be based on the same geodetic datum, which requires the correct
* ellipsoid the associated with any given geodetic datum.</P>
*
* <P ALIGN="justify">An ellipsoid is defined either by its semi-major axis and
* inverse flattening, or by its semimajor axis and semi-minor axis. For some
* applications, for example small-scale mapping in atlases, a spherical approximation
* of the geoid's surface is used, requiring only the radius of the sphere to be
* specified.</P>
*
* @version <A HREF="http://portal.opengeospatial.org/files/?artifact_id=6716">Abstract specification 2.0</A>
* @since GeoAPI 1.0
*/
package org.opengis.referencing.datum;