empty geometries

[edzer]

GDAL seems to return empty geoms as GEOMETRYCOLLECTION with zero elements:

> st_difference(st_sfc(st_point(0:1),st_point(c(0,0))), st_sfc(st_point(0:1)))
Geometry set for 2 features 
geometry type:  GEOMETRYCOLLECTION
dimension:      XY
bbox:           xmin: 0 ymin: 0 xmax: 0 ymax: 0
epsg (SRID):    NA
proj4string:    NA
precision:      double (default; no precision model)
GEOMETRYCOLLECTION() 
GEOMETRYCOLLECTION(POINT(0 0)) 

The current logic of sfr is then to convert everything back to GEOMETRYCOLLECTION, only because the empty one is. Having empty points doesn't seem a good solution, as they don't have a WKB representation. Maybe tweak sfc such that they have a constant type (in this case POINT), but may contain empty geometries of type GEOMETRYCOLLECTION?

[edzer]

Is an empty point semantically different from an empty polygon?

[edzer]

see tests/empty.R, all geometries now allow empty, also from/to WKB (except for POINT, but this seems an omission in the standard; GPKG uses silent NA values for empty POINT geoms); also added sfc_GEOMETRY type to mix anything.