org.opengis.referencing.datum.package-info Maven / Gradle / Ivy
/*
* 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 OpenGIS® Spatial Referencing
* by Coordinates (Topic 2) specification.
*
* 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.
*
*
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.
*
*
*
*
Vertical datum
*
* Further sub-typing is required to describe vertical datums adequately. The
* following types of vertical datum are distinguished:
*
*
* -
*
Geoidal
* 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.
*
-
*
Depth
* 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.
*
-
*
Barometric
* 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.
*
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".
*
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.
*
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.
*
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).
*
-
*
Other surface
* 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.
*
*
*
*
*
Image datum
*
* 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".
*
*
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.
*
*
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".
*
*
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".
*
*
This difference in perspective has no effect on the image interpretation, but
* is important for coordinate transformations involving this defined image.
*
*
*
*
Prime meridian
*
* 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.
*
*
*
*
Ellipsoid
*
* 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.
*
*
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.
*
*
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.
*
* @version Abstract
* specification 2.0
* @since GeoAPI 1.0
*/
package org.opengis.referencing.datum;