Point (2D) Expands to Point Z on Intersection with LineString ZM

[jlhumber]

Hi geos team,

When working with a 2D point (just XY) and intersecting with a LineStringZM (or PolygonZM), the resulting Point is a Point Z whereas we were expecting it to expand out to a Point ZM like is done with other geometries, for example this is the happy path for a LineStringZM intersected with a LineString

from shapely import from_wkt
linezm = from_wkt('LineString (0 0 100 200, 10 0 300 400)')
line = from_wkt('LineString (2 0, 8 0)')
linezm.intersection(line)
# <LINESTRING ZM (2 0 140 240, 8 0 260 360)>

The above shows that the outcome includes ZM, in the case of a Point however this is what we see:

from shapely import from_wkt
linezm = from_wkt('LineString (0 0 100 200, 10 0 300 400)')
p = from_wkt('Point (2 0)')
p.intersection(linezm)
# <POINT Z (2 0 100)>

No measure dimension on the Point, Point Z is used for intersection there also is no M value added to the result, only when a Point ZM is used will the result include M when used on a LineString ZM (or Polygon ZM)

Note also the interpolated value for Z is different than that interpolated for a LineString (this will be reported in another issue).

ref: shapely/shapely#2404

[dbaston]

I've looked into this one and found two problems:

• The dimension of points created by OverlayMixedPoints is XYZ, regardless of the dimensionality of the input. This is easily changed by dbaston@8ada588
• Missing Z values in result points are populated using the ElevationModel rather than interpolation along the line. The ElevationModel is not applied to the M dimension, so the above commit only gets us to a result of POINT ZM (2 0 100 NaN).