geof:projectTo description

[FransKnibbe]

The function geof:projectTo assumes that GeoSPARQL supports 3D geometry, but that is not the case:

geof:projectTo (geom1:  ogc:geomLiteral,
                dimVec: xsd:integer): ogc:geomLiteral

Projects the elements of geometry geom1 to the dimensions specified by the dimVec "dimensional vector".

dimVec is a binary number indicating which dimensions to project to (retain) from the geom, for example, using dimensions x, y & z:

• "010" projects to the y dimension, only
• "110" projects to the x and y dimensions

A projection may only occur from a geometry to a geometry of equal or fewer dimensions and, so far, GeoSPARQL only allos 1-, 2- & 3-dimensional objects, thus projectTo can only be specified from 3 to 2, 2 to 1 or 1 to 0 dimensions.

[FransKnibbe]

To clarify, this function sets all coordinates on the position of the 0 to 0, right?

Why not have a binary filter of four positions, including M-coordinates? M and Z-coordinates have roughly the same status in GeoSPARQL; they are ignored. So the same reason for zeroing of Z-coordinates could apply to M-coordinates. The function name should be changed in that case though, but maybe that needs to be done anyway to avoid confusion with projecting from a curved to a flat plane.

projectTo can only be specified from 3 to 2, 2 to 1 or 1 to 0 dimensions

Why should only one coordinate be zeroed? The first example goes from 3 to 1 dimension(s).

[jabhay]

The projectTo name is a bit misleading. The behaviour that we want to support is allowing someone to alter the number of dimensions for which there are values in the geometry. We need more information in the definition to help people understand what happens when they supply a 0 in the dimVec string, and what happens if they supply a 1 in the dimVec string when the original geometry doesn't have a corresponding value.

A better name for this function could be: alterDimensions.

For now though, it's not necessary for 1.1 and our preference is to make it a consideration for 1.2. Agreement of the SWG it to remove it for now.