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+-------------------------------------------------------------------------
+
+TITLE: ZM values and SRID for Simple Features
+
+AUTHOR: Name: Sandro Santilli
+ Email: strk@refractions.net
+
+DATE: 27 December 2005
+
+CATEGORY: Simple Features Revision Proposal
+
+-------------------------------------------------------------------------
+
+1. Background
+
+OpenGIS document 99-402r2 introduces semantic and well-known
+representations for Z-geometries. This proposal extend the well-known
+representations to optionally also hold a measure (M) and a SRID.
+Misures, as Z values, are attributes of 2D vertexes, but their
+semantic is unspecified in this document, as they could be used
+for any kind 'misurement'. SRID is an attribute of the whole feature.
+
+This document defines how geometry can have Z,M or both values and SRID
+in a way which is compatible to the existing 2D OpenGIS Simple Features
+specification AND to the Z-Geometry documented in OpenGIS 99-402r2.
+
+2. Proposal
+
+2.1. Definition of ZM-Geometry
+
+a) A geometry can have either 2, 3 or 4 dimensions.
+b) 3rd dimension of a 3d geometry can either represent Z or M (3DZ or 3DM).
+c) 4d geometries contain both Z and M (in this order).
+d) M and Z values are associated with every vertex.
+e) M and Z values are undefined within surface interiors.
+
+Any ZM-Geometry can be converted into a 2D geometry by discarding all its
+Z and M values. The resulting 2D geometry is the "shadow" of the ZM-Geometry.
+2D geometries cannot be safely converted into ZM-Geometries, since their Z
+and M values are undefined, and not necessarily zero.
+
+2.2. Extensions to Well-Known-Binary format
+
+The 2d OpenGIS Simple Features specification has the following geometry types:
+
+enum wkbGeometryType {
+ wkbPoint = 1,
+ wkbLineString = 2,
+ wkbPolygon = 3,
+ wkbMultiPoint = 4,
+ wkbMultiLineString = 5,
+ wkbMultiPolygon = 6,
+ wkbGeometryCollection = 7
+}
+
+Document 99-402r2 introduces a Z-presence flag (wkbZ) which OR'ed
+to the type specifies the presence of Z coordinate:
+
+ wkbZ = 0x80000000
+
+This proposal suggest the use of an M-presence flag (wkbM) to
+allow for XY, XYM, XYZ and XYZM geometryes, and SRID-presence
+flag to allow for embedded SRID:
+
+ wkbM = 0x40000000
+ wkbSRID = 0x20000000
+
+Possible resulting geometry types are:
+
+enum wkbGeometryTypeZ {
+
+ wkbPoint = 1,
+ wkbLineString = 2,
+ wkbPolygon = 3,
+ wkbMultiPoint = 4,
+ wkbMultiLineString = 5,
+ wkbMultiPolygon = 6,
+ wkbGeometryCollection = 7,
+
+ // | 0x80000000
+ wkbPointZ = 0x80000001,
+ wkbLineStringZ = 0x80000002,
+ wkbPolygonZ = 0x80000003,
+ wkbMultiPointZ = 0x80000004,
+ wkbMultiLineStringZ = 0x80000005,
+ wkbMultiPolygonZ = 0x80000006,
+ wkbGeometryCollectionZ = 0x80000007,
+
+ // | 0x40000000
+ wkbPointM = 0x40000001,
+ wkbLineStringM = 0x40000002,
+ wkbPolygonM = 0x40000003,
+ wkbMultiPointM = 0x40000004,
+ wkbMultiLineStringM = 0x40000005,
+ wkbMultiPolygonM = 0x40000006,
+ wkbGeometryCollectionM = 0x40000007,
+
+ // | 0x40000000 | 0x80000000
+ wkbPointZM = 0xC0000001,
+ wkbLineStringZM = 0xC0000002,
+ wkbPolygonZM = 0xC0000003,
+ wkbMultiPointZM = 0xC0000004,
+ wkbMultiLineStringZM = 0xC0000005,
+ wkbMultiPolygonZM = 0xC0000006,
+ wkbGeometryCollectionZM = 0xC0000007,
+
+ // | 0x20000000
+ wkbPointS = 0x20000001,
+ wkbLineStringS = 0x20000002,
+ wkbPolygonS = 0x20000003,
+ wkbMultiPointS = 0x20000004,
+ wkbMultiLineStringS = 0x20000005,
+ wkbMultiPolygonS = 0x20000006,
+ wkbGeometryCollectionS = 0x20000007,
+
+ // | 0x20000000 | 0x80000000
+ wkbPointZS = 0xA0000001,
+ wkbLineStringZS = 0xA0000002,
+ wkbPolygonZS = 0xA0000003,
+ wkbMultiPointZS = 0xA0000004,
+ wkbMultiLineStringZS = 0xA0000005,
+ wkbMultiPolygonZS = 0xA0000006,
+ wkbGeometryCollectionZS = 0xA0000007,
+
+ // | 0x20000000 | 0x40000000
+ wkbPointMS = 0x60000001,
+ wkbLineStringMS = 0x60000002,
+ wkbPolygonMS = 0x60000003,
+ wkbMultiPointMS = 0x60000004,
+ wkbMultiLineStringMS = 0x60000005,
+ wkbMultiPolygonMS = 0x60000006,
+ wkbGeometryCollectionMS = 0x60000007,
+
+ // | 0x20000000 | 0x40000000 | 0x80000000
+ wkbPointZMS = 0xE0000001,
+ wkbLineStringZMS = 0xE0000002,
+ wkbPolygonZMS = 0xE0000003,
+ wkbMultiPointZMS = 0xE0000004,
+ wkbMultiLineStringZMS = 0xE0000005,
+ wkbMultiPolygonZMS = 0xE0000006,
+ wkbGeometryCollectionZMS = 0xE0000007,
+}
+
+
+If the SRID flag is set it's value is encoded as a 4byte integer
+right after the type integer.
+
+If only wkbZ or wkbM flags are set Point coordinates will
+be XYZ or XYM, if both wkbZ and wkbM flags are set Point
+coordinates will be XYZM (Z first).
+
+For example, a ZM-Point geometry at the location (10,20) with Z==30,
+M==40 and SRID=4326 would be:
+
+WKBPoint {
+
+ byte byteOrder; // wkbXDR or wkbNDR
+
+ uint32 wkbType; // (wkbPoint+wkbZ+wkbM+wkbSRID) =
+ // 0xE0000001
+
+ uint32 SRID; // 4326
+
+ Point {
+ Double x; // 10.0
+ Double y; // 20.0
+ Double z; // 30.0
+ Double m; // 40.0
+ }
+}
+
+
+2.3. Extensions to Well-Known-Text format
+
+Geometry SRID presence and value would be represented using a
+"SRID=#;" prefix to the WKT text:
+
+ "SRID=4326;POINT(1 2)"
+
+3DZ geometry will be represented as:
+
+ "POINT(1 2 3)"
+
+4D geometry will be represented as:
+
+ "POINT(1 2 3 4)"
+
+3DM geometry will be represented as:
+
+ "POINTM(1 2 3)"
+ or
+ "GEOMETRYCOLLECTIONM(POINTM(1 2 3), LINESTRINGM(1 2 3, 4 5 6))"
+
+Note that the coordinates structure of a geometry must be consistent,
+you can't mix dimensions in a single geometry.
+
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