GML Subset schema for gml:_Feature,gml:_FeatureCollection,gml:FeaturePropertyType,gml:AbstractFeatureType,gml:AssociationAttributeGroup,gml:featureMember,gml:AbstractFeatureCollectionType,gml:metaDataProperty,gml:description,gml:name,gml:boundedBy,gml:_MetaData,gml:AbstractMetaDataType,gml:PointPropertyType,gml:Point,gml:pos,gml:pointRep,gml:coordinates,gml:DirectedNodePropertyType,gml:Node,gml:pointProperty,gml:container,gml:directedEdge,gml:CurvePropertyType,gml:Curve,gml:segments,gml:LineString,gml:LineStringSegment,gml:Arc,gml:CubicSpline,gml:DirectedEdgePropertyType,gml:Edge,gml:directedNode,gml:directedFace,gml:curveProperty,gml:SurfacePropertyType,gml:Polygon,gml:Ring,gml:LinearRing,gml:exterior,gml:interior,gml:curveMember,gml:DirectedFacePropertyType,gml:Face,gml:surfaceProperty,gml:isolated,gml:SolidPropertyType,gml:Solid,gml:DirectedTopoSolidPropertyType,gml:TopoSolid, written by gmlSubset.xslt. An abstract feature provides a set of common properties, including id, metaDataProperty, name and description inherited from AbstractGMLType, plus boundedBy. A concrete feature type must derive from this type and specify additional properties in an application schema. A feature must possess an identifying attribute ('id' - 'fid' has been deprecated). deprecated deprecated in GML version 3.1 All complexContent GML elements are directly or indirectly derived from this abstract supertype to establish a hierarchy of GML types that may be distinguished from other XML types by their ancestry. Elements in this hierarchy may have an ID and are thus referenceable. This content model group makes it easier to construct types that derive from AbstractGMLType and its descendents "by restriction". A reference to the group saves having to enumerate the standard object properties. Multiple names may be provided. These will often be distinguished by being assigned by different authorities, as indicated by the value of the codeSpace attribute. In an instance document there will usually only be one name per authority. Contains or refers to a metadata package that contains metadata properties. Base type for complex metadata property types. Attribute group used to enable property elements to refer to their value remotely. It contains the simple link components from xlinks.xsd, with all members optional, and the remoteSchema attribute, which is also optional. These attributes can be attached to any element, thus allowing it to act as a pointer. The 'remoteSchema' attribute allows an element that carries link attributes to indicate that the element is declared in a remote schema rather than by the schema that constrains the current document instance. Reference to an XML Schema fragment that specifies the content model of the propertys value. This is in conformance with the XML Schema Section 4.14 Referencing Schemas from Elsewhere. Contains a simple text description of the object, or refers to an external description. This type is available wherever there is a need for a "text" type property. It is of string type, so the text can be included inline, but the value can also be referenced remotely via xlinks from the AssociationAttributeGroup. If the remote reference is present, then the value obtained by traversing the link should be used, and the string content of the element can be used for an annotation. Label for the object, normally a descriptive name. An object may have several names, typically assigned by different authorities. The authority for a name is indicated by the value of its (optional) codeSpace attribute. The name may or may not be unique, as determined by the rules of the organization responsible for the codeSpace. Name or code with an (optional) authority. Text token. If the codeSpace attribute is present, then its value should identify a dictionary, thesaurus or authority for the term, such as the organisation who assigned the value, or the dictionary from which it is taken. A text string with an optional codeSpace attribute. Database handle for the object. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs. An external identifier for the object in the form of a URI may be constructed using standard XML and XPointer methods. This is done by concatenating the URI for the document, a fragment separator, and the value of the id attribute. Bounding shape. Envelope defines an extent using a pair of positions defining opposite corners in arbitrary dimensions. The first direct position is the "lower corner" (a coordinate position consisting of all the minimal ordinates for each dimension for all points within the envelope), the second one the "upper corner" (a coordinate position consisting of all the maximal ordinates for each dimension for all points within the envelope). deprecated deprecated with GML version 3.0 deprecated Deprecated with GML version 3.1. Use the explicit properties "lowerCorner" and "upperCorner" instead. Deprecated with GML version 3.1.0. Use the explicit properties "lowerCorner" and "upperCorner" instead. DirectPosition instances hold the coordinates for a position within some coordinate reference system (CRS). Since DirectPositions, as data types, will often be included in larger objects (such as geometry elements) that have references to CRS, the "srsName" attribute will in general be missing, if this particular DirectPosition is included in a larger element with such a reference to a CRS. In this case, the CRS is implicitly assumed to take on the value of the containing object's CRS. XML List based on XML Schema double type. An element of this type contains a space-separated list of double values Optional reference to the CRS used by this geometry, with optional additional information to simplify use when a more complete definition of the CRS is not needed. In general this reference points to a CRS instance of gml:CoordinateReferenceSystemType (see coordinateReferenceSystems.xsd). For well known references it is not required that the CRS description exists at the location the URI points to. If no srsName attribute is given, the CRS must be specified as part of the larger context this geometry element is part of, e.g. a geometric element like point, curve, etc. It is expected that this attribute will be specified at the direct position level only in rare cases. The "srsDimension" is the length of coordinate sequence (the number of entries in the list). This dimension is specified by the coordinate reference system. When the srsName attribute is omitted, this attribute shall be omitted. Optional additional and redundant information for a CRS to simplify use when a more complete definition of the CRS is not needed. This information shall be the same as included in the more complete definition of the CRS, referenced by the srsName attribute. When the srsName attribute is included, either both or neither of the axisLabels and uomLabels attributes shall be included. When the srsName attribute is omitted, both of these attributes shall be omitted. Ordered list of labels for all the axes of this CRS. The gml:axisAbbrev value should be used for these axis labels, after spaces and forbiddden characters are removed. When the srsName attribute is included, this attribute is optional. When the srsName attribute is omitted, this attribute shall also be omitted. Ordered list of unit of measure (uom) labels for all the axes of this CRS. The value of the string in the gml:catalogSymbol should be used for this uom labels, after spaces and forbiddden characters are removed. When the axisLabels attribute is included, this attribute shall also be included. When the axisLabels attribute is omitted, this attribute shall also be omitted. A set of values, representing a list of token with the lexical value space of NCName. The tokens are seperated by whitespace. Deprecated with GML 3.0 and included for backwards compatibility with GML 2. Use the "pos" element instead. Represents a coordinate tuple in one, two, or three dimensions. Deprecated with GML 3.0 and replaced by DirectPositionType. Deprecated with GML version 3.1.0. Tables or arrays of tuples. May be used for text-encoding of values from a table. Actually just a string, but allows the user to indicate which characters are used as separators. The value of the 'cs' attribute is the separator for coordinate values, and the value of the 'ts' attribute gives the tuple separator (a single space by default); the default values may be changed to reflect local usage. Defaults to CSV within a tuple, space between tuples. However, any string content will be schema-valid. Utility type for null elements. The value may be selected from one of the enumerated tokens, or may be a URI in which case this should identify a resource which describes the reason for the null. Some common reasons for a null value: innapplicable - the object does not have a value missing - The correct value is not readily available to the sender of this data. Furthermore, a correct value may not exist. template - the value will be available later unknown - The correct value is not known to, and not computable by, the sender of this data. However, a correct value probably exists. withheld - the value is not divulged other:reason - as indicated by "reason" string Specific communities may agree to assign more strict semantics when these terms are used in a particular context. Deprecated in GML 3.1.0 Convenience property for generalised location. A representative location for plotting or analysis. Often augmented by one or more additional geometry properties with more specific semantics. Deprecated in GML 3.1.0 The "_Geometry" element is the abstract head of the substituition group for all geometry elements of GML 3. This includes pre-defined and user-defined geometry elements. Any geometry element must be a direct or indirect extension/restriction of AbstractGeometryType and must be directly or indirectly in the substitution group of "_Geometry". All geometry elements are derived directly or indirectly from this abstract supertype. A geometry element may have an identifying attribute ("gml:id"), a name (attribute "name") and a description (attribute "description"). It may be associated with a spatial reference system (attribute "srsName"). The following rules shall be adhered: - Every geometry type shall derive from this abstract type. - Every geometry element (i.e. an element of a geometry type) shall be directly or indirectly in the substitution group of _Geometry. This attribute is included for backward compatibility with GML 2 and is deprecated with GML 3. This identifer is superceded by "gml:id" inherited from AbstractGMLType. The attribute "gid" should not be used anymore and may be deleted in future versions of GML without further notice. Global element which acts as the head of a substitution group that may include any element which is a GML feature, object, geometry or complex value This abstract element is the head of a substitutionGroup hierararchy which may contain either simpleContent or complexContent elements. It is used to assert the model position of "class" elements declared in other GML schemas. A feature collection contains zero or more features. Container for a feature - follow gml:AssociationType pattern. Container for features - follow gml:ArrayAssociationType pattern. Abstract element which acts as the head of a substitution group for packages of MetaData properties. An abstract base type for complex metadata types. A property that has a point as its value domain can either be an appropriate geometry element encapsulated in an element of this type or an XLink reference to a remote geometry element (where remote includes geometry elements located elsewhere in the same document). Either the reference or the contained element must be given, but neither both nor none. This attribute group includes the XLink attributes (see xlinks.xsd). XLink is used in GML to reference remote resources (including those elsewhere in the same document). A simple link element can be constructed by including a specific set of XLink attributes. The XML Linking Language (XLink) is currently a Proposed Recommendation of the World Wide Web Consortium. XLink allows elements to be inserted into XML documents so as to create sophisticated links between resources; such links can be used to reference remote properties. A simple link element can be used to implement pointer functionality, and this functionality has been built into various GML 3 elements by including the gml:AssociationAttributeGroup. A Point is defined by a single coordinate tuple. GML supports two different ways to specify the direct poisiton of a point. 1. The "pos" element is of type DirectPositionType. Deprecated with GML version 3.1.0 for coordinates with ordinate values that are numbers. Use "pos" instead. The "coordinates" element shall only be used for coordinates with ordinates that require a string representation, e.g. DMS representations. Deprecated with GML version 3.0. Use "pos" instead. The "coord" element is included for backwards compatibility with GML 2. This is the abstract root type of the geometric primitives. A geometric primitive is a geometric object that is not decomposed further into other primitives in the system. All primitives are oriented in the direction implied by the sequence of their coordinate tuples. The "_GeometricPrimitive" element is the abstract head of the substituition group for all (pre- and user-defined) geometric primitives. Deprecated with GML version 3.1.0. Use "pointProperty" instead. Included for backwards compatibility with GML 3.0.0. Its optional co-boundary is a set of connected directedEdges. The orientation of one of these dirEdges is "+" if the Node is the "to" node of the Edge, and "-" if it is the "from" node. There is precisely one positively directed and one negatively directed node in the boundary of every edge. The negatively and positively directed nodes correspond to the start and end nodes respectively. The optional coboundary of an edge is a circular sequence of directed faces which are incident on this edge in document order. Faces which use a particular boundary edge in its positive orientation appear with positive orientation on the coboundary of the same edge. In the 2D case, the orientation of the face on the left of the edge is "+"; the orientation of the face on the right on its right is "-". An edge may optionally be realised by a 1-dimensional (curve) geometric primitive. . The topological boundary of a face consists of a set of directed edges. Note that all edges associated with a Face, including dangling and interior edges, appear in the boundary. Dangling and interior edges are each referenced by pairs of directedEdges with opposing orientations. The optional coboundary of a face is a pair of directed solids which are bounded by this face. If present, there is precisely one positively directed and one negatively directed solid in the coboundary of every face. The positively directed solid corresponds to the solid which lies in the direction of the positively directed normal to the face in any geometric realisation. A face may optionally be realised by a 2-dimensional (surface) geometric primitive. Utility type used in various places - e.g. to indicate the direction of topological objects; "+" for forwards, or "-" for backwards. The topological boundary of a TopoSolid consists of a set of directed faces. Note that all faces associated with the TopoSolid, including dangling faces, appear in the boundary. The coboundary of a TopoSolid is empty and hence requires no representation. Substitution group branch for Topo Primitives, used by TopoPrimitiveArrayAssociationType This property element either references a surface via the XLink-attributes or contains the surface element. surfaceProperty is the predefined property which can be used by GML Application Schemas whenever a GML Feature has a property with a value that is substitutable for _Surface. A property that has a surface as its value domain can either be an appropriate geometry element encapsulated in an element of this type or an XLink reference to a remote geometry element (where remote includes geometry elements located elsewhere in the same document). Either the reference or the contained element must be given, but neither both nor none. This attribute group includes the XLink attributes (see xlinks.xsd). XLink is used in GML to reference remote resources (including those elsewhere in the same document). A simple link element can be constructed by including a specific set of XLink attributes. The XML Linking Language (XLink) is currently a Proposed Recommendation of the World Wide Web Consortium. XLink allows elements to be inserted into XML documents so as to create sophisticated links between resources; such links can be used to reference remote properties. A simple link element can be used to implement pointer functionality, and this functionality has been built into various GML 3 elements by including the gml:AssociationAttributeGroup. The "_Surface" element is the abstract head of the substituition group for all (continuous) surface elements. An abstraction of a surface to support the different levels of complexity. A surface is always a continuous region of a plane. This property element either references a curve via the XLink-attributes or contains the curve element. curveProperty is the predefined property which can be used by GML Application Schemas whenever a GML Feature has a property with a value that is substitutable for _Curve. A property that has a curve as its value domain can either be an appropriate geometry element encapsulated in an element of this type or an XLink reference to a remote geometry element (where remote includes geometry elements located elsewhere in the same document). Either the reference or the contained element must be given, but neither both nor none. This attribute group includes the XLink attributes (see xlinks.xsd). XLink is used in GML to reference remote resources (including those elsewhere in the same document). A simple link element can be constructed by including a specific set of XLink attributes. The XML Linking Language (XLink) is currently a Proposed Recommendation of the World Wide Web Consortium. XLink allows elements to be inserted into XML documents so as to create sophisticated links between resources; such links can be used to reference remote properties. A simple link element can be used to implement pointer functionality, and this functionality has been built into various GML 3 elements by including the gml:AssociationAttributeGroup. The "_Curve" element is the abstract head of the substituition group for all (continuous) curve elements. An abstraction of a curve to support the different levels of complexity. The curve can always be viewed as a geometric primitive, i.e. is continuous. This property element either references a point via the XLink-attributes or contains the point element. pointProperty is the predefined property which can be used by GML Application Schemas whenever a GML Feature has a property with a value that is substitutable for Point. Curve is a 1-dimensional primitive. Curves are continuous, connected, and have a measurable length in terms of the coordinate system. A curve is composed of one or more curve segments. Each curve segment within a curve may be defined using a different interpolation method. The curve segments are connected to one another, with the end point of each segment except the last being the start point of the next segment in the segment list. The orientation of the curve is positive. This element encapsulates the segments of the curve. This property element contains a list of curve segments. The order of the elements is significant and shall be preserved when processing the array. A container for an array of curve segments. The "_CurveSegment" element is the abstract head of the substituition group for all curve segment elements, i.e. continuous segments of the same interpolation mechanism. Curve segment defines a homogeneous segment of a curve. The attribute "numDerivativesAtStart" specifies the type of continuity between this curve segment and its predecessor. If this is the first curve segment in the curve, one of these values, as appropriate, is ignored. The default value of "0" means simple continuity, which is a mandatory minimum level of continuity. This level is referred to as "C 0 " in mathematical texts. A value of 1 means that the function and its first derivative are continuous at the appropriate end point: "C 1 " continuity. A value of "n" for any integer means the function and its first n derivatives are continuous: "C n " continuity. NOTE: Use of these values is only appropriate when the basic curve definition is an underdetermined system. For example, line string segments cannot support continuity above C 0 , since there is no spare control parameter to adjust the incoming angle at the end points of the segment. Spline functions on the other hand often have extra degrees of freedom on end segments that allow them to adjust the values of the derivatives to support C 1 or higher continuity. The attribute "numDerivativesAtEnd" specifies the type of continuity between this curve segment and its successor. If this is the last curve segment in the curve, one of these values, as appropriate, is ignored. The default value of "0" means simple continuity, which is a mandatory minimum level of continuity. This level is referred to as "C 0 " in mathematical texts. A value of 1 means that the function and its first derivative are continuous at the appropriate end point: "C 1 " continuity. A value of "n" for any integer means the function and its first n derivatives are continuous: "C n " continuity. NOTE: Use of these values is only appropriate when the basic curve definition is an underdetermined system. For example, line string segments cannot support continuity above C 0 , since there is no spare control parameter to adjust the incoming angle at the end points of the segment. Spline functions on the other hand often have extra degrees of freedom on end segments that allow them to adjust the values of the derivatives to support C 1 or higher continuity. The attribute "numDerivativesInterior" specifies the type of continuity that is guaranteed interior to the curve. The default value of "0" means simple continuity, which is a mandatory minimum level of continuity. This level is referred to as "C 0 " in mathematical texts. A value of 1 means that the function and its first derivative are continuous at the appropriate end point: "C 1 " continuity. A value of "n" for any integer means the function and its first n derivatives are continuous: "C n " continuity. NOTE: Use of these values is only appropriate when the basic curve definition is an underdetermined system. For example, line string segments cannot support continuity above C 0 , since there is no spare control parameter to adjust the incoming angle at the end points of the segment. Spline functions on the other hand often have extra degrees of freedom on end segments that allow them to adjust the values of the derivatives to support C 1 or higher continuity. A LineString is a special curve that consists of a single segment with linear interpolation. It is defined by two or more coordinate tuples, with linear interpolation between them. It is backwards compatible with the LineString of GML 2, GM_LineString of ISO 19107 is implemented by LineStringSegment. GML supports two different ways to specify the control points of a line string. 1. A sequence of "pos" (DirectPositionType) or "pointProperty" (PointPropertyType) elements. "pos" elements are control points that are only part of this curve, "pointProperty" elements contain a point that may be referenced from other geometry elements or reference another point defined outside of this curve (reuse of existing points). 2. The "posList" element allows for a compact way to specifiy the coordinates of the control points, if all control points are in the same coordinate reference systems and belong to this curve only. The number of direct positions in the list must be at least two. Deprecated with GML version 3.1.0. Use "pointProperty" instead. Included for backwards compatibility with GML 3.0.0. Deprecated with GML version 3.0. Use "pos" instead. The "coord" element is included for backwards compatibility with GML 2. Deprecated with GML version 3.1.0. Use "posList" instead. DirectPositionList instances hold the coordinates for a sequence of direct positions within the same coordinate reference system (CRS). "count" allows to specify the number of direct positions in the list. If the attribute count is present then the attribute srsDimension shall be present, too. A LineStringSegment is a curve segment that is defined by two or more coordinate tuples, with linear interpolation between them. Note: LineStringSegment implements GM_LineString of ISO 19107. GML supports two different ways to specify the control points of a curve segment. 1. A sequence of "pos" (DirectPositionType) or "pointProperty" (PointPropertyType) elements. "pos" elements are control points that are only part of this curve segment, "pointProperty" elements contain a point that may be referenced from other geometry elements or reference another point defined outside of this curve segment (reuse of existing points). 2. The "posList" element allows for a compact way to specifiy the coordinates of the control points, if all control points are in the same coordinate reference systems and belong to this curve segment only. The number of direct positions in the list must be at least two. Deprecated with GML version 3.1.0. Use "pointProperty" instead. Included for backwards compatibility with GML 3.0.0. Deprecated with GML version 3.1.0. Use "posList" instead. The attribute "interpolation" specifies the curve interpolation mechanism used for this segment. This mechanism uses the control points and control parameters to determine the position of this curve segment. For a LineStringSegment the interpolation is fixed as "linear". CurveInterpolationType is a list of codes that may be used to identify the interpolation mechanisms specified by an application schema. An Arc is an arc string with only one arc unit, i.e. three control points. GML supports two different ways to specify the control points of a curve segment. 1. A sequence of "pos" (DirectPositionType) or "pointProperty" (PointPropertyType) elements. "pos" elements are control points that are only part of this curve segment, "pointProperty" elements contain a point that may be referenced from other geometry elements or reference another point defined outside of this curve segment (reuse of existing points). 2. The "posList" element allows for a compact way to specifiy the coordinates of the control points, if all control points are in the same coordinate reference systems and belong to this curve segment only. The number of direct positions in the list must be three. Deprecated with GML version 3.1.0. Use "pointProperty" instead. Included for backwards compatibility with GML 3.0.0. Deprecated with GML version 3.1.0. Use "posList" instead. An arc is an arc string consiting of a single arc, the attribute is fixed to "1". An ArcString is a curve segment that uses three-point circular arc interpolation. GML supports two different ways to specify the control points of a curve segment. 1. A sequence of "pos" (DirectPositionType) or "pointProperty" (PointPropertyType) elements. "pos" elements are control points that are only part of this curve segment, "pointProperty" elements contain a point that may be referenced from other geometry elements or reference another point defined outside of this curve segment (reuse of existing points). 2. The "posList" element allows for a compact way to specifiy the coordinates of the control points, if all control points are in the same coordinate reference systems and belong to this curve segment only. The number of direct positions in the list must be at least three. Deprecated with GML version 3.1.0. Use "pointProperty" instead. Included for backwards compatibility with GML 3.0.0. Deprecated with GML version 3.1.0. Use "posList" instead. The attribute "interpolation" specifies the curve interpolation mechanism used for this segment. This mechanism uses the control points and control parameters to determine the position of this curve segment. For an ArcString the interpolation is fixed as "circularArc3Points". The number of arcs in the arc string can be explicitly stated in this attribute. The number of control points in the arc string must be 2 * numArc + 1. Cubic splines are similar to line strings in that they are a sequence of segments each with its own defining function. A cubic spline uses the control points and a set of derivative parameters to define a piecewise 3rd degree polynomial interpolation. Unlike line-strings, the parameterization by arc length is not necessarily still a polynomial. The function describing the curve must be C2, that is, have a continuous 1st and 2nd derivative at all points, and pass through the controlPoints in the order given. Between the control points, the curve segment is defined by a cubic polynomial. At each control point, the polynomial changes in such a manner that the 1st and 2nd derivative vectors are the same from either side. The control parameters record must contain vectorAtStart, and vectorAtEnd which are the unit tangent vectors at controlPoint[1] and controlPoint[n] where n = controlPoint.count. Note: only the direction of the vectors is relevant, not their length. GML supports two different ways to specify the control points of a curve segment. 1. A sequence of "pos" (DirectPositionType) or "pointProperty" (PointPropertyType) elements. "pos" elements are control points that are only part of this curve segment, "pointProperty" elements contain a point that may be referenced from other geometry elements or reference another point defined outside of this curve segment (reuse of existing points). 2. The "posList" element allows for a compact way to specifiy the coordinates of the control points, if all control points are in the same coordinate reference systems and belong to this curve segment only. The number of direct positions in the list must be at least three. Deprecated with GML version 3.1.0. Use "pointProperty" instead. Included for backwards compatibility with GML 3.0.0. Deprecated with GML version 3.1.0. Use "posList" instead. "vectorAtStart" is the unit tangent vector at the start point of the spline. "vectorAtEnd" is the unit tangent vector at the end point of the spline. The attribute "interpolation" specifies the curve interpolation mechanism used for this segment. This mechanism uses the control points and control parameters to determine the position of this curve segment. For a CubicSpline the interpolation is fixed as "cubicSpline". The degree for a cubic spline is "3". Vector instances hold the compoents for a (usually spatial) vector within some coordinate reference system (CRS). Since Vectors will often be included in larger objects that have references to CRS, the "srsName" attribute may be missing. In this case, the CRS is implicitly assumed to take on the value of the containing object's CRS. Note that this content model is the same as DirectPositionType, but is defined separately to reflect the distinct semantics, and to avoid validation problems. SJDC 2004-12-02 A Polygon is a special surface that is defined by a single surface patch. The boundary of this patch is coplanar and the polygon uses planar interpolation in its interior. It is backwards compatible with the Polygon of GML 2, GM_Polygon of ISO 19107 is implemented by PolygonPatch. A boundary of a surface consists of a number of rings. In the normal 2D case, one of these rings is distinguished as being the exterior boundary. In a general manifold this is not always possible, in which case all boundaries shall be listed as interior boundaries, and the exterior will be empty. Encapsulates a ring to represent the surface boundary property of a surface. The "_Ring" element is the abstract head of the substituition group for all closed boundaries of a surface patch. An abstraction of a ring to support surface boundaries of different complexity. A boundary of a surface consists of a number of rings. The "interior" rings seperate the surface / surface patch from the area enclosed by the rings. A Ring is used to represent a single connected component of a surface boundary. It consists of a sequence of curves connected in a cycle (an object whose boundary is empty). A Ring is structurally similar to a composite curve in that the endPoint of each curve in the sequence is the startPoint of the next curve in the Sequence. Since the sequence is circular, there is no exception to this rule. Each ring, like all boundaries, is a cycle and each ring is simple. NOTE: Even though each Ring is simple, the boundary need not be simple. The easiest case of this is where one of the interior rings of a surface is tangent to its exterior ring. This element references or contains one curve in the composite curve. The curves are contiguous, the collection of curves is ordered. NOTE: This definition allows for a nested structure, i.e. a CompositeCurve may use, for example, another CompositeCurve as a curve member. This property element either references a curve via the XLink-attributes or contains the curve element. A curve element is any element which is substitutable for "_Curve". A LinearRing is defined by four or more coordinate tuples, with linear interpolation between them; the first and last coordinates must be coincident. GML supports two different ways to specify the control points of a linear ring. 1. A sequence of "pos" (DirectPositionType) or "pointProperty" (PointPropertyType) elements. "pos" elements are control points that are only part of this ring, "pointProperty" elements contain a point that may be referenced from other geometry elements or reference another point defined outside of this ring (reuse of existing points). 2. The "posList" element allows for a compact way to specifiy the coordinates of the control points, if all control points are in the same coordinate reference systems and belong to this ring only. The number of direct positions in the list must be at least four. Deprecated with GML version 3.1.0. Use "pointProperty" instead. Included for backwards compatibility with GML 3.0.0. Deprecated with GML version 3.1.0. Use "posList" instead. Deprecated with GML version 3.0 and included for backwards compatibility with GML 2. Use "pos" elements instead. A property that has a solid as its value domain can either be an appropriate geometry element encapsulated in an element of this type or an XLink reference to a remote geometry element (where remote includes geometry elements located elsewhere in the same document). Either the reference or the contained element must be given, but neither both nor none. This attribute group includes the XLink attributes (see xlinks.xsd). XLink is used in GML to reference remote resources (including those elsewhere in the same document). A simple link element can be constructed by including a specific set of XLink attributes. The XML Linking Language (XLink) is currently a Proposed Recommendation of the World Wide Web Consortium. XLink allows elements to be inserted into XML documents so as to create sophisticated links between resources; such links can be used to reference remote properties. A simple link element can be used to implement pointer functionality, and this functionality has been built into various GML 3 elements by including the gml:AssociationAttributeGroup. The "_Solid" element is the abstract head of the substituition group for all (continuous) solid elements. An abstraction of a solid to support the different levels of complexity. A solid is always contiguous. A solid is the basis for 3-dimensional geometry. The extent of a solid is defined by the boundary surfaces (shells). A shell is represented by a composite surface, where every shell is used to represent a single connected component of the boundary of a solid. It consists of a composite surface (a list of orientable surfaces) connected in a topological cycle (an object whose boundary is empty). Unlike a Ring, a Shell's elements have no natural sort order. Like Rings, Shells are simple. Boundaries of solids are similar to surface boundaries. In normal 3-dimensional Euclidean space, one (composite) surface is distinguished as the exterior. In the more general case, this is not always possible. Boundaries of solids are similar to surface boundaries.