[GRASS-user] Varying results - v.select,v.distance,v.what

[Markus Neteler]

On Mon, Dec 15, 2008 at 9:07 PM, Kevin Webb <kfw4 at cornell.edu> wrote:
> At 03:03 PM 11/20/2008, Kevin Webb wrote:
...
>> --The layer for which v.select, v.distance, and v.what results do NOT
>> match--
>>
>
> I think the issue with varying results for v.select, v.distance , and v.what as I
> have experienced are due to an OGR 'relationship with' the GEOS library.
> Tests I have run suggest the GEOS library should be an installation
> requirement/prerequisite
> for any user intending on executing operations on vectors imported using
> v.external.
> (The GEOS library may be an undocumented requirement for all OGR ops.)
>
> Note: I used the phrase "relationship with'" instead of "dependency". OGR
> will certainly
> compile without GEOS, but OGR operations without GEOS may yield suspect
> results.

Isn't it then an OGR problem rather than a GRASS problem?
We use the OGR and expect that OGR always delivers the same
result (or issues a warning/error if not possible in absence of GEOS).

> This discovery came about when I was testing code that calls the OGR library
> directly.
> Calls to OGRGeometry::Distance(OGRGeometry *) errored out because GEOS was
> not installed. After installing GEOS and recompiling GDAL,
> OGRGeometry::Distance() works
> and I noticed changes in values for my test points previously run with
> OGRGeometry::Intersects().
>
> Thinking that GEOS may have an impact on Grass, I executed similar
> before-and-after-GEOS
> tests in Grass and observed the same improvement; points that resulted in
> multiple FIDs
> coming from v.select before-GEOS, yielded a single value after-GEOS.
> v.select and v.distance
> return the same value now that GEOS has been installed.

May I ask you to bring this up on the gdal mailing list?

Markus