Sarah, > I'm having trouble making convex hulls that cross the international dateline. Imagine 4 points making a square with the international dateline running down the middle--I would like the convex hull to be that square, but instead I get a giant rectangle that goes across the entire planet. I've done some reading, and it sounds like GEOS generally has a hard time dealing with the dateline--is that accurate? Yes GEOS only deals with planar coordinates. Geography is the only geodetic aware type and there is no ConvexHull function for it. > Does anyone know of a way to get around this? I was thinking that it might work if I use an SRID that is just like 4326 but with a central meridian of 180, does that sound like a good plan? No. You should use a planar projection of some sort. 4326 squashed on a map is no good. Paul Ramsey might have some thoughts on the matter. > I'm pretty new to postGIS and I'm not sure how to either find such an SRID or how to define it--I've been trying to find an explanation of SRID syntax and so far coming up empty. If anyone has advice for solving this problem, or for places where I can learn more about defining custom SRIDs, I'd be really grateful! You might want to check out http://spatialreference.org
> Here's an example--if you make this table and then look at it in QGIS (or whatever) along with a world map, you'll see a big rectangle spanning the entire map. On a map with > central meridian of zero, I'd want to see half the polygon on the left side of the map and the other half on the right side. > CREATE TABLE example AS > SELECT ST_ConvexHull( > ST_Collect(ST_GeomFromText('MULTIPOINT(175 5, 175 30, -175 5, -175 30)') ))::geography(Polygon, 4326) ; My guess is you'll have to cut your area into pieces. Still then its not that pretty when you try to rejoin. Sorry couldn't be more help, Regina http://www.postgis.us
_______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users