Will, I could be way off base here because I don't think too much about geometries except in my little world so maybe someone can correct me if my assumptions are way off base.
Topological relations and processes like ST_Contains, ST_Intersection, ST_Intersects are operations that are always valid regardless of if your data is projected or unprojected. If a contains b in WGS 84 long lat space, a will contain b in any projection you can project them to (and vice versa). I think about these things as just wrapping rubber bands around stuff. No matter how you distort the shape of the rubber band, things inside it are always inside it. Now as far as indexes go - they wouldn't be as efficient in non-projected space because a box in unprojected has less meaning (since its a planar thing in spherical space). So your operation will work but it will be slower since you'll get more false positives. An extent will always contain the geometry in question regardless of projection (although the extent of unprojected would be different from if you had it in planar). Regardless - all objects contained within an object should always have extents that intersect the extent of the object that contains them in any space. So my thought process tells me, you shouldn't get any points left out, you'll just get more false positives from the && operation and thus slower performance. So just keep your data in SRID 4326 do your contains in that and see how slow it is. May not be all that slow. Hope that helps, Regina -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of William Temperley Sent: Friday, June 27, 2008 4:49 AM To: postgis-users@postgis.refractions.net Subject: [postgis-users] ST_Contains - WGS84 Hi all Is there currently an accurate way to run queries like ST_Contains on geodetic data? My dataset is global and I may have to perform queries on a continental scale, e.g. find all the points in Africa. I don't think I can reproject to the UTM zone in question, given the queries will span several zones. Is there a projection I can use temporarily to perform queries like this? I'm also slightly worried that the planar nature of the spatial indexes could prematurely remove points from the result set - is this the case? Will _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users ----------------------------------------- The substance of this message, including any attachments, may be confidential, legally privileged and/or exempt from disclosure pursuant to Massachusetts law. It is intended solely for the addressee. If you received this in error, please contact the sender and delete the material from any computer. _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
