datums.xsd
See ISO/DIS 19136 13.5
The datums schema components can be divided into three logical parts, which define elements and types for XML encoding of the definitions of:
- Abstract datum
- Geodetic datums, including ellipsoid and prime meridian
- Multiple other concrete types of spatial or temporal datums
These schema components encode the Datum packages of the UML Models of ISO 19111 Clause 10 and ISO/DIS 19136 D.3.10.
A gml:AbstractDatum specifies the relationship of a coordinate system to the earth, thus creating a coordinate reference system. A datum uses a parameter or set of parameters that determine the location of the origin of the coordinate reference system. Each datum subtype may be associated with only specific types of coordinate systems. This abstract complex type shall not be used, extended, or restricted, in a GML Application Schema, to define a concrete subtype with a meaning equivalent to a concrete subtype specified in this document.
gml:anchorDefinition is a description, possibly including coordinates, of the definition used to anchor the datum to the Earth. Also known as the "origin", especially for engineering and image datums. The codeSpace attribute may be used to reference a source of more detailed on this point or surface, or on a set of such descriptions.
- For a geodetic datum, this point is also known as the fundamental point, which is traditionally the point where the relationship between geoid and ellipsoid is defined. In some cases, the "fundamental point" may consist of a number of points. In those cases, the parameters defining the geoid/ellipsoid relationship have been averaged for these points, and the averages adopted as the datum definition.
- For an engineering datum, the anchor definition may be a physical point, or it may be a point with defined coordinates in another CRS.may
- For an image datum, the anchor definition is usually either the centre of the image or the corner of the image.
- For a temporal datum, this attribute is not defined. Instead of the anchor definition, a temporal datum carries a separate time origin of type DateTime.
gml:realizationEpoch is the time after which this datum definition is valid. See ISO 19111 Table 32 for details.
gml:DatumPropertyType is a property type for association roles to a datum, either referencing or containing the definition of that datum.
gml:GeodeticDatum is a geodetic datum defines the precise location and orientation in 3-dimensional space of a defined ellipsoid (or sphere), or of a Cartesian coordinate system centered in this ellipsoid (or sphere).
gml:primeMeridian is an association role to the prime meridian used by this geodetic datum.
gml:ellipsoid is an association role to the ellipsoid used by this geodetic datum.
gml:GeodeticDatumPropertyType is a property type for association roles to a geodetic datum, either referencing or containing the definition of that datum.
A gml:Ellipsoid is a geometric figure that may be used to describe the approximate shape of the earth. In mathematical terms, it is a surface formed by the rotation of an ellipse about its minor axis.
gml:semiMajorAxis specifies the length of the semi-major axis of the ellipsoid, with its units. Uses the MeasureType with the restriction that the unit of measure referenced by uom must be suitable for a length, such as metres or feet.
gml:secondDefiningParameter is a property containing the definition of the second parameter that defines the shape of an ellipsoid. An ellipsoid requires two defining parameters: semi-major axis and inverse flattening or semi-major axis and semi-minor axis. When the reference body is a sphere rather than an ellipsoid, only a single defining parameter is required, namely the radius of the sphere; in that case, the semi-major axis "degenerates" into the radius of the sphere.
The inverseFlattening element contains the inverse flattening value of the ellipsoid. This value is a scale factor (or ratio). It uses gml:LengthType with the restriction that the unit of measure referenced by the uom attribute must be suitable for a scale factor, such as percent, permil, or parts-per-million.
The semiMinorAxis element contains the length of the semi-minor axis of the ellipsoid. When the isSphere element is included, the ellipsoid is degenerate and is actually a sphere. The sphere is completely defined by the semi-major axis, which is the radius of the sphere.
gml:EllipsoidPropertyType is a property type for association roles to an ellipsoid, either referencing or containing the definition of that ellipsoid.
A gml:PrimeMeridian defines the origin from which longitude values are determined. The default value for the prime meridian gml:identifier value is "Greenwich".
gml:greenwichLongitude is the longitude of the prime meridian measured from the Greenwich meridian, positive eastward. If the value of the prime meridian “name” is "Greenwich" then the value of greenwichLongitude shall be 0 degrees.
gml:PrimeMeridianPropertyType is a property type for association roles to a prime meridian, either referencing or containing the definition of that meridian.
gml:ImageDatum defines the origin of an image coordinate reference system, and is used in a local context only. For an image datum, the anchor definition is usually either the centre of the image or the corner of the image. For more information, see ISO 19111 B.3.5.
gml:pixelInCell is a specification of the way an image grid is associated with the image data attributes. The required codeSpace attribute shall reference a source of information specifying the values and meanings of all the allowed string values for this property.
gml:ImageDatumPropertyType is a property type for association roles to an image datum, either referencing or containing the definition of that datum.
gml:VerticalDatum is a textual description and/or a set of parameters identifying a particular reference level surface used as a zero-height surface, including its position with respect to the Earth for any of the height types recognized by this International Standard.
gml:VerticalDatumPropertyType is property type for association roles to a vertical datum, either referencing or containing the definition of that datum.