Hi, I need to do the following request to find some data:
- I have a bunch of towns (let's call them group A) - I retrieve the towns touching the group A (let's call them group B) - I make the union of group A and group B - I ask the towns covered by the resulting union For the moment, I use GDAL to do the processing like that: - I use OGRGeometry::Union to make the union of group A towns - I send a request to a postgis database with a ST_Touches where clause and the group A geometry as WKT - I make the union of the resulting geometries with OGRGeometry::Union - I send a request to a postgis database with a ST_Covers where clause and the union of group A and group B as WKT My problem is that I don't retrieve all the towns I initially have to build my geometry. If I try to do the same request totally in pgsql (union, touches and covers), I have the awaited result. If I compare the area of the geometry of the ST_Covers where clause (between generation with GDAL and computation by postgis), I ha ve the same area. What could explain the difference in the result ? Precision loss in GDAL ? Precision loss because of the use of WKT ? Should I use WKB instead of WKT ? Should I do all my computations in postgis ? -- Alexandre Gacon
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