gml:TopoSolid represents the 3-dimensional topology primitive. The topological boundary of a solid (gml:directedFace) consists of a set of directed faces.A solid may optionally be realised by a 3-dimensional geometric primitive (gml:solidProperty).
The attribute gml:id supports provision of a handle for the XML element representing a GML Object. Its use is mandatory for all GML objects. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs.
A gml:TopoSolid must indicate whether it is a universal topo-solid or not, to ensure a lossless topology representation as defined by Kuijpers, et. al. (see OGC 05-102 Topology IPR). The optional universal attribute of type boolean is used to indicate this and the default is fault. NOTE The universal topo-solid is normally not part of any feature, and is used to represent the unbounded portion of the data set. Its interior boundary (it has no exterior boundary) would normally be considered the exterior boundary of the data set.
Source
<element name="TopoSolid" type="gml:TopoSolidType" substitutionGroup="gml:AbstractTopoPrimitive"><annotation><documentation>gml:TopoSolid represents the 3-dimensional topology primitive. The topological boundary of a solid (gml:directedFace) consists of a set of directed faces. A solid may optionally be realised by a 3-dimensional geometric primitive (gml:solidProperty).</documentation></annotation></element>