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			datums.xsd
			3.0
			How to encode datum definitions. 
			Builds on referenceSystems.xsd to encode the data needed to define datums, including the specific subtypes of datums. Primary editor: Arliss Whiteside.
			
				GML is an OGC Standard.
				Copyright (c) 2001,2002,2010 Open Geospatial Consortium.
				To obtain additional rights of use, visit http://www.opengeospatial.org/legal/ .
			
			This schema encodes the Datum (CD_) package of the extended UML Model for OGC Abstract Specification Topic 2: Spatial Referencing by Coordinates. That UML model is adapted from ISO 19111 - Spatial referencing by coordinates, as described in Annex B of Topic 2. 
		
	
	
	
	
	
	
	
		
			Basic encoding for datum objects, simplifying and restricting the DefinitionType as needed. 
		
		
			
				
					
					
						
							Comments on this reference system, including source information. 
						
					
					
				
				
			
		
	
	
	
		
			The name by which this datum is identified. 
		
	
	
	
		
			A datum 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, the orientation, and the scale of a coordinate reference system. This abstract complexType shall not be used, extended, or restricted, in an Application Schema, to define a concrete subtype with a meaning equivalent to a concrete subtype specified in this document. 
		
		
			
				
					
					
					
					
					
				
			
		
	
	
	
		
			An identification of a datum. The first datumID, if any, is normally the primary identification code, and any others are aliases. 
		
	
	
	
		
			Description, possibly including coordinates, of the point or points used to anchor the datum to the Earth. Also known as the "origin", especially for engineering and image datums. The codeSpace attribute can 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 point may be a physical point, or it may be a point with defined coordinates in another CRS.
- For an image datum, the anchor point 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 point, a temporal datum carries a separate time origin of type DateTime. 
		
	
	
	
		
			The time for which this datum definition is valid. This time may be precise (e.g. 1997.0 for IRTF97) or merely a year (e.g. 1983 for NAD83). In the latter case, the epoch usually refers to the year in which a major recalculation of the geodetic control network, underlying the datum, was executed or initiated. 
		
	
	
	
	
	
		
			Association to a datum, either referencing or containing the definition of that datum. 
		
		
			
				
					
				
				
			
		
	
	
	
	
	
		
			An engineering datum defines the origin and axes directions of an engineering coordinate reference system. Normally used in a local context only. 
		
		
			
		
	
	
	
	
	
		
			Association to an engineering datum, either referencing or containing the definition of that datum. 
		
		
			
				
					
				
				
			
		
	
	
	
	
	
		
			An image datum defines the origin of an image coordinate reference system. Used in a local context only. For more information, see OGC Abstract Specification Topic 2. 
		
		
			
				
					
				
			
		
	
	
	
	
	
		
			Specification of the way an image grid is associated with the image data attributes. 
		
		
			
				
					
						Reference to a source of information specifying the values and meanings of all the allowed string values for this PixelInCellType. 
					
				
			
		
	
	
	
	
	
		
			Association to an image datum, either referencing or containing the definition of that datum. 
		
		
			
				
					
				
				
			
		
	
	
	
	
	
		
			A textual description and/or a set of parameters identifying a particular reference level surface used as a zero-height surface, including its position and orientation with respect to the Earth for any of the height types recognized by this standard. 
		
		
			
				
					
				
			
		
	
	
	
	
	
		
			Type of a vertical datum. 
		
		
			
				
					
						Reference to a source of information specifying the values and meanings of all the allowed string values for this VerticalDatumTypeType. 
					
				
			
		
	
	
	
	
	
		
			Association to a vertical datum, either referencing or containing the definition of that datum. 
		
		
			
				
					
				
				
			
		
	
	
	
	
	
		
			Partially defines the origin of a temporal coordinate reference system. This type restricts the AbstractDatumType to remove the "anchorPoint" and "realizationEpoch" elements. 
		
		
			
				
					
					
					
					
					
				
				
			
		
	
	
	
		
			A temporal datum defines the origin of a temporal coordinate reference system. This type extends the TemporalDatumRestrictionType to add the "origin" element with the dateTime type. 
		
		
			
				
					
				
			
		
	
	
	
		
			The date and time origin of this temporal datum. 
		
	
	
	
	
	
		
			Association to a temporal datum, either referencing or containing the definition of that datum. 
		
		
			
				
					
				
				
			
		
	
	
	
	
	
		
			A geodetic datum defines the 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). 
		
		
			
				
					
					
				
			
		
	
	
	
		
			Association to the prime meridian used by this geodetic datum. 
		
	
	
	
		
			Association to the ellipsoid used by this geodetic datum. 
		
	
	
	
	
	
		
			Association to a geodetic datum, either referencing or containing the definition of that datum. 
		
		
			
				
					
				
				
			
		
	
	
	
	
	
	
		
			Basic encoding for prime meridian objects, simplifying and restricting the DefinitionType as needed. 
		
		
			
				
					
					
						
							Comments on or information about this prime meridian, including source information. 
						
					
					
				
				
			
		
	
	
	
		
			The name by which this prime meridian is identified. 
		
	
	
	
		
			A prime meridian defines the origin from which longitude values are determined.
		
		
			
				
					
					
				
			
		
	
	
	
		
			An identification of this prime meridian. The first meridianID, if any, is normally the primary identification code, and any others are aliases. 
		
	
	
	
		
			Longitude of the prime meridian measured from the Greenwich meridian, positive eastward. If the datum type is geodetic and the prime meridian name is not supplied, then the prime meridian name is taken to be "Greenwich" and the Greenwich longitude value is taken to be zero. 
		
	
	
	
	
	
		
			Association to a prime meridian, either referencing or containing the definition of that meridian. 
		
		
			
				
					
				
				
			
		
	
	
	
	
	
		
			Basic encoding for ellipsoid objects, simplifying and restricting the DefinitionType as needed. 
		
		
			
				
					
					
						
							Comments on or information about this ellipsoid, including source information. 
						
					
					
				
				
			
		
	
	
	
		
			The name by which this ellipsoid is identified. 
		
	
	
	
		
			An ellipsoid is a geometric figure that can 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.
		
		
			
				
					
					
					
				
			
		
	
	
	
		
			An identification of this ellipsoid. The first ellipsoidID, if any, is normally the primary identification code, and any others are aliases. 
		
	
	
	
		
			Length of the semi-major axis of the ellipsoid. 
		
	
	
	
	
	
		
			Association to an ellipsoid, either referencing or containing the definition of that ellipsoid. 
		
		
			
				
					
				
				
			
		
	
	
	
	
	
		
			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.
		
		
			
			
			
		
	
	
	
		
			Inverse flattening value of the ellipsoid. 
		
	
	
	
		
			Length of the semi-minor axis of the ellipsoid. 
		
	
	
	
		
			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.