[postgis-devel] ST_Union for TopoGeometry (misnamer?)

[Paul Ramsey]

What does the resulting TopoGeom look like? If I put together a topogeom with A areas and one with B areas do I end up with a new one with > A+B areas? that's a "coverage union" in Esri terms, but more like an "overlay" in the words we usually use in postgis land. A "union" of sets of areas should result in fewer areas (assuming the areas are not all disjoint).

Helpful?

P

> On Feb 17, 2021, at 11:13 AM, Sandro Santilli <strk at kbt.io> wrote:
> 
> Hi all,
> I've been working on a function for adding the components
> of a topologically defined geometry (src TopoGeometry) to the
> components of another one (tgt TopoGeometry).
> 
> In my current draft [1] this function has signature:
> 
>  toTopoGeom(src TopoGeometry, tgt TopoGeometry)
> 
> But when approaching documenting it, I figured that "toTopoGeom"
> is documented to _convert_ simple geometries to TopoGeometris
> while in this function there's no conversion at all.
> 
> So, the question is: would you find ST_Union [2] an appropriate
> name for the function, even if it would raise an exception
> when trying to union "incompatible" TopoGeometries (defined on
> different topologies or different hierarchies?)
> 
> That is: is it acceptable for ST_Union(tg1, tg2) to fail
> when ST_Union(tg1::geometry, tg2::geometry) would succeed ?
> 
> Your brain power is welcome :)
> 
> [1] https://gitlab.com/postgis/postgis/-/merge_requests/20
> [2] https://trac.osgeo.org/postgis/ticket/4847
> 
> --strk; 
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