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

Sarah Berke skberke at uchicago.edu
Tue Sep 6 10:13:19 PDT 2011

Hello,

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? 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?
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!

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) ;

Thanks very much,
Sarah

_________________________________
Sarah K Berke
Postdoctoral Scholar
Department of the Geophysical Sciences
University of Chicago
5734 S. Ellis Ave
Chicago, IL 60637
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