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.
Application Data
datums.xsd More information at: urn:ogc:specification:gml:schema-xsd:datums:3.2.0.
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.
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:EngineeringDatum defines the origin of an engineering coordinate reference system, and is used in a region around that origin. This origin may be fixed with respect to the earth (such as a defined point at a construction site), or be a defined point on a moving vehicle (such as on a ship or satellite).
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: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.
A gml:PrimeMeridian defines the origin from which longitude values are determined. The default value for the prime meridian gml:identifier value is "Greenwich".
A gml:TemporalDatum defines the origin of a Temporal Reference System. This type omits the "anchorDefinition" and "realizationEpoch" elements and adds the "origin" element with the dateTime type.
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: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: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: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: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: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:EngineeringDatumPropertyType is a property type for association roles to an engineering datum, either referencing or containing the definition of that datum.
gml:GeodeticDatumPropertyType is a property type for association roles to a geodetic datum, either referencing or containing the definition of that datum.
gml:PrimeMeridianPropertyType is a property type for association roles to a prime meridian, either referencing or containing the definition of that meridian.
The TemporalDatumBaseType partially defines the origin of a temporal coordinate reference system. This type restricts the AbstractDatumType to remove the "anchorDefinition" and "realizationEpoch" elements.
gml:TemporalDatumPropertyType is a property type for association roles to a temporal datum, either referencing or containing the definition of that datum.
gml:VerticalDatumPropertyType is property type for association roles to a vertical datum, either referencing or containing the definition of that datum.
The XML Instance Representation table above shows the schema component's content as an XML instance.
The minimum and maximum occurrence of elements and attributes are provided in square brackets, e.g. [0..1].
Model group information are shown in gray, e.g. Start Choice ... End Choice.
For type derivations, the elements and attributes that have been added to or changed from the base type's content are shown in bold.
If an element/attribute has a fixed value, the fixed value is shown in green, e.g. country="Australia".
Otherwise, the type of the element/attribute is displayed.
If the element/attribute's type is in the schema, a link is provided to it.
For local simple type definitions, the constraints are displayed in angle brackets, e.g. <<pattern = [1-9][0-9]{3}>>.
If a local element/attribute has documentation, it will be displayed in a window that pops up when the question mark inside the attribute or next to the element is clicked, e.g. <postcode>.
Abstract(Applies to complex type definitions and element declarations). An abstract element or complex type cannot used to validate an element instance. If there is a reference to an abstract element, only element declarations that can substitute the abstract element can be used to validate the instance. For references to abstract type definitions, only derived types can be used.
Collapse Whitespace PolicyReplace tab, line feed, and carriage return characters with space character (Unicode character 32). Then, collapse contiguous sequences of space characters into single space character, and remove leading and trailing space characters.
Disallowed Substitutions(Applies to element declarations). If substitution is specified, then substitution group members cannot be used in place of the given element declaration to validate element instances. If derivation methods, e.g. extension, restriction, are specified, then the given element declaration will not validate element instances that have types derived from the element declaration's type using the specified derivation methods. Normally, element instances can override their declaration's type by specifying an xsi:type attribute.
Nillable(Applies to element declarations). If an element declaration is nillable, instances can use the xsi:nil attribute. The xsi:nil attribute is the boolean attribute, nil, from the http://www.w3.org/2001/XMLSchema-instance namespace. If an element instance has an xsi:nil attribute set to true, it can be left empty, even though its element declaration may have required content.
Prohibited Derivations(Applies to type definitions). Derivation methods that cannot be used to create sub-types from a given type definition.
Prohibited Substitutions(Applies to complex type definitions). Prevents sub-types that have been derived using the specified derivation methods from validating element instances in place of the given type definition.
Replace Whitespace PolicyReplace tab, line feed, and carriage return characters with space character (Unicode character 32).
Substitution GroupElements that are members of a substitution group can be used wherever the head element of the substitution group is referenced.
Substitution Group Exclusions(Applies to element declarations). Prohibits element declarations from nominating themselves as being able to substitute a given element declaration, if they have types that are derived from the original element's type using the specified derivation methods.
Target NamespaceThe target namespace identifies the namespace that components in this schema belongs to. If no target namespace is provided, then the schema components do not belong to any namespace.