WKB representation of POINT EMPTY has 3 dimensions

[Mike Taves]

This is related to #1005

An empty point geometry can be created using either POINT EMPTY or with GEOSGeom_createEmptyPoint_r. This geometry returns 0 from GEOSHasZ_r and the WKT representation is POINT EMPTY, as expected.

However, the WKB representation is 0101000080000000000000F87F000000000000F87F000000000000F87F or POINT Z EMPTY. The expected result should be 0101000000000000000000F87F000000000000F87F.

The only workaround is to use GEOSWKBWriter_setOutputDimension_r to write a 2D representation.

Note that WKB representations of MULTIPOINT EMPTY or GEOSGeom_createEmptyLineString_r do not contain Z coordinate info, for any WKB writer output dimension setting.

[Mike Taves]

Also, unsure if this is directly related, but WKT GEOMETRYCOLLECTION (POINT EMPTY) returns WKB 0107000080010000000101000080000000000000F87F000000000000F87F000000000000F87F or GEOMETRYCOLLECTION Z (POINT Z EMPTY)

Whereas GEOMETRYCOLLECTION (MULTIPOINT EMPTY) remains a 2D GeometryCollection with 010700000001000000010400000000000000

[Mike Taves]

Another related inconsistency GEOMETRYCOLLECTION (POINT(1 2), POINT EMPTY) returns 0107000080020000000101000000000000000000F03F00000000000000400101000080000000000000F87F000000000000F87F000000000000F87F, which is not portable WKB. Handing this to PostGIS raises "Dimensions mismatch in lwcollection", because of the extra coordinate surprise.

[pramsey]

In general the WKT reader and default creators try to create things with 3d coordinates, which is counter to my own sense of "least surprise". Directly relating to this ticket, the EMPTY case in WKT reader fails to note the presence of any dimensionality tags in the input (POINT Z EMPTY) and just creates a 3d empty every time. Looks like most types will need to be touched to put this completely right.

[pramsey]

In general the WKT parser did not parse in "GEOMETRY Z EMPTY" forms and notice there was a Z there. Point was fiddly to fix, and that still left other types, so I figured I'd fix them all... turns out the core model of GEOS/JTS can't even represent a "GEOMETRYCOLLECTION Z EMPTY" of any type. There's nowhere for the dimensionality to live!

Also! The question of what to instantiate for a "POINT EMPTY", dimensionally speaking. The default in the code was a 3D empty. That seemed wrong to me, so I've moved it down to a 2D empty. That should also cause other things to harmonize. There was also a bug hiding, in the clone() method, that incorrectly cloned 2D POINT EMPTY.

Anyways, just waiting for a CI build before merging, but suffice to say this isn't a *comprehensive* fix, since that requires some larger restructuring to even be able to represent some of these concepts that are expressible in WKT and WKB.

[Paul Ramsey <pramsey@…>]

In 708d32b/git:

Create new EMPTY geometry with 2D coordinates by default. Parse Z EMPTY geometry in WKT reader, for simple (non-multi) geometries. Closes #1048