[gdal-dev] OGR question on geometric computations

[Adriano Ferreira]

Hi, I am working with GDAL Swig interface to Perl and have a doubt I
could not resolve by myself from what I read up to now. (Warning: This
is a newbye question.)

I am working with polygons defined with lat/long points according to
NAD83 (more specifically, EPSG:4269 as SRS). I need to do some
computations with them and also with points given as input. The
GDAL/OGR methods like Contains, Overlaps, etc. are pretty handy and
easy to use. But what geometry does it use? Does it make difference?

I am not sure that in set operations like Contains, Overlaps, etc. it
does make difference if a simple Euclidian geometry used in the (lat,
long) pairs will give me a different answer than if using a geometry
over the spherical approximation of the Earth. And I think it does not
(though I should re-read some math books before taking it for sure :-)
).

But my concern is with the Distance method. What does it compute?
Euclidean distance? It can compute great circle distance if I use the
correct Spatial Reference objects with the OGR objects?

I am not sure this question is clear: but as an example, given an
polygon "P" built of (lat, long) points (given in degrees) and a a
point "pt", the return of

       P.Distance(pt)

is just the length of the minimum Euclidean length between the polygon
points and "pt"?

Thanks for any help.

Adriano Ferreira