Clarify the scope of delta values for geometries in a GeometryCollection

[airbreather]

https://github.com/TWKB/Specification/blob/351dd1e1de0629a9dc2e49dab26bf70faea93b8c/twkb.md#calculate-delta-values says this:

Every coordinate value in a geometry except the very first one is a delta value relative to the previous value processed.

https://github.com/TWKB/Specification/blob/351dd1e1de0629a9dc2e49dab26bf70faea93b8c/twkb.md#geometrycollection-type-7 says this:

• for each geometry there will be a complete TWKB geometry, readable using the rules set out above

This creates an apparent ambiguity: if a GeometryCollection contains a Polygon followed by a Linestring, then is the first coordinate of the Linestring treated as:

• a delta from the last coordinate of the last ring of the preceding Polygon (because they're both part of the same GeometryCollection, which is itself "a geometry"), or
• the actual point value (because otherwise it would not count as "a complete TWKB geometry")

[nicklasaven]

I see your point

This was also discussed when we designed the format.

The result was that the subgeometries in a geomoetrycollection is "complete twkb geometries". In other words, each subgeometry has it's own absolute starting value.

But for multigeometries like multipoint, multiline and multipolygon it is only one absolute value in the beginning.

This can be seen in the PostGIS-implementation:

SELECT 
length(
   st_astwkb(
      'multipoint((1000000 1000000),(1000001 1000001))'::geometry
      )
   )

which gives a twkb length of 11 bytes while the corresponding geoemtry collection:

SELECT 
length(
   st_astwkb(
      'geometrycollection(point(1000000 1000000),point(1000001 1000001))'::geometry
      )
   )

gives a twkb-length of 19 bytes.

The reason was if I recall correct mostly was that it is easier to implement this way. When creating or reading a geometry-collection it just iterates a bunch of geometries that is complete. The function for reading a polygon from a geometrycollection is the same as the function reading a plain polygon.

I have added a note in the specification to clearify. Does this help?

[airbreather]

I have added a note in the specification to clearify. Does this help?

Yes, thanks for the clarification. Closing.