[postgis-users] Problem with convex hulls that cross the dateline

[Paragon Corporation]

 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
 
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