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

[Ben Madin]

Sarah,

I'm sure that there are good reasons not to do this, but could you translate your points left or right, create your convex hull then translate the polygon back...?

Otherwise you could project it onto a custom projection that covers your area of interest? As an example look for an equal area proj string (I know there is one for Australia GDA94 Albers - http://www.spatialreference.org/ref/epsg/3577) and shift the lat and lon parameters...

cheers

Ben


On 07/09/2011, at 1:13 AM, Sarah Berke wrote:

> 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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