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===PostGIS Serialized Geometry Format===
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This document describe the serialied form of
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Geometries as implemented starting from version 1.x.
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Prior versions used a different format.
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The new format is aimed at reducing geometries size, for
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this reason we refer to the old format as HWGEOM (High Weight GEOMetry)
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and to the new one as LWGEOM (Light Weight GEOMetry).
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==Serialized Form and PostgreSQL varlena objects==
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PostGIS serialized form is a recursive structure, where
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elements in a collection (multi* or geometrycollections)
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are complete serialized forms themself.
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PostgreSQL requires to add an header to variable-lenght
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user defined types, for this reason, the type actually
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stored on disk is a SERIALIZED geometry wrapped in this
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required header. We call the complete PostgreSQL type
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==Geometry Type byte==
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All serialiezd geometries start with a single byte
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encoding geometry type (lower nibble) and flags
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B = 16 byte BOX2DFLOAT4 follows (probably not aligned) [before SRID]
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S = 4 byte SRID attached (0= not attached (-1), 1= attached)
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ZM = dimensionality (hasZ, hasM)
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wkbMultiLineString = 5
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wkbGeometryCollection = 7
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* Note that the following values do not correspond to wkb type values.
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==Bounding box and SRID==
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Following the Geometry type byte we find optional bounding box
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and/or SRID values. Presence of them is encoded in the type byte.
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If both objects are present the bounding box come first.
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Bounding box is composed by 4 32-bit float:
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[FLOAT32] [FLOAT32] [FLOAT32] [FLOAT32]
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SRID is composed by a 32-bit integer:
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When it comes to ordinate values all geometries use arrays of 64-bit
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floats. Number and semantic of values depend on the dimension flags
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on the geometry type byte:
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[FLOAT64] [FLOAT64] [FLOAT64]
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[FLOAT64] [FLOAT64] [FLOAT64]
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[FLOAT64] [FLOAT64] [FLOAT64] [FLOAT64]
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What you find after the type byte and optional bbox and SRID
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(COMMON_HEADER from now on) depends on actual geometry type.
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A Point geometry is as follows:
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<ORDINATE_ARRAY> -- single element
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A LineString geometry is as follows:
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<UINT32> -- number of elements in following ORDINATE_ARRAY
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A CircularString geometry is as follows:
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<UINT32> -- number of elements in following ORDINATE_ARRAY
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A CompoundString geometry is as follows:
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<UINT32> -- number of segments
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One or more geometries as specified by previous uint32:
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<SERIALIZED_GEOMETRY> -- Must be some combination of
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LineStrings and CircularStrings.
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A Polygon geometry is as follows:
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<UINT32> -- number of ORDINATE_ARRAYs (at least 1, the shell)
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One or more arrays (rings) as specified by previous uint32:
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<UINT32> -- number of elements in following ORDINATE_ARRAY
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<ORDINATE_ARRAY> -- single element
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A CurvePolygon geometry is as follows:
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<UINT32> -- number of rings
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One or more geometries as specified by previous uint32:
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<SERIALIZED_GEOMETRY> -- Must be some combination of
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LineStrings and CircularStrings.
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CompoundString are not yet supported.
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All collection types (MultiPoint, MultiLineString, MultiPolygon,
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GeometryCollection) have the same structure. Real type is just
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an hint on what one should expect from element geometries.
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Note that the GeometryCollection type is also used for EMPTY
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Geometries (number_of_geometries is set to 0 in this case).
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A Collection geometry is as follows:
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<UINT32> -- number of GEOMETRIES
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Zero (EMPTY) or more geometries as specified by previous uint32:
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<SERIALIZED_GEOMETRY> -- starting from type byte again
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=3DZ point, no bounding box, no SRID=
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<char> TYPE - flags:__Z_ type:1
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=3DM point, bounding box, no SRID=
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<char> TYPE - flags:B__M type:1
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=2D LineString, no bounding box, SRID=
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<char> TYPE - flags:_S__ type:2
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=2D polygon, no bounding box, no SRID=
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<char> TYPE - flags:____ type:3
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=2D Collection of a point and a line, SRID and bbox present=
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<char> TYPE - flags:BS__ type:7
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<char> TYPE - flags:____ type:1 ( the point )
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<char> TYPE - flags:____ type:2 ( the line )
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Note that in the inner geometries:
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- SRID value is inherited by parent geometry type.
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- BBOX value is not present (can be, however)
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- ZM flags must exactly match those in the outer geometry type byte
added
removed
postgis/SERIALIZED_FORM
