Thanks very much, Regina, Paul, and Ben for the responses. I don't think my hacking skills are quite up to following Paul's suggestion, but I can try to find an equal area projection for the Pacific Ocean. It seems like that would work. I also rather like the idea of translating the points--if I did that, would it then be easy to just translate the entire hull back into position?<br>
<br>Thanks,<br>Sarah<br clear="all">_________________________________<br>Sarah K Berke<br>Postdoctoral Scholar<br>Department of the Geophysical Sciences<br>University of Chicago<br>5734 S. Ellis Ave<br>Chicago, IL 60637<br>
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Message: 2<br>
Date: Wed, 7 Sep 2011 01:18:51 -0400<br>
From: "Paragon Corporation" <<a href="mailto:lr@pcorp.us">lr@pcorp.us</a>><br>
Subject: Re: [postgis-users] Problem with convex hulls that cross the<br>
dateline<br>
To: "'PostGIS Users Discussion'"<br>
<<a href="mailto:postgis-users@postgis.refractions.net">postgis-users@postgis.refractions.net</a>><br>
Message-ID: <F8675FD4BB254CD09D2359CF11685DEA@D><br>
Content-Type: text/plain; charset="us-ascii"<br>
<br>
Sarah,<br>
> I'm having trouble making convex hulls that cross the international<br>
dateline. Imagine 4 points making a square with the international dateline<br>
running down the middle--I would like the convex hull to be that square, but<br>
instead I get a giant rectangle that goes across the entire planet. I've<br>
done some reading, and it sounds like GEOS generally has a hard time dealing<br>
with the dateline--is that accurate?<br>
Yes GEOS only deals with planar coordinates. Geography is the only<br>
geodetic aware type and there is no ConvexHull function for it.<br>
<br>
> Does anyone know of a way to get around this? I was thinking that it<br>
might work if I use an SRID that is just like 4326 but with a central<br>
meridian of 180, does that sound like a good plan?<br>
No. You should use a planar projection of some sort. 4326 squashed on a<br>
map is no good. Paul Ramsey might have some thoughts on the matter.<br>
<br>
> I'm pretty new to postGIS and I'm not sure how to either find such an<br>
SRID or how to define it--I've been trying to find an explanation of SRID<br>
syntax and so far coming up empty. If anyone has advice for solving this<br>
problem, or for places where I can learn more about defining custom SRIDs,<br>
I'd be really grateful!<br>
<br>
You might want to check out <a href="http://spatialreference.org" target="_blank">http://spatialreference.org</a><br>
<br>
> Here's an example--if you make this table and then look at it in QGIS<br>
(or whatever) along with a world map, you'll see a big rectangle spanning<br>
the entire map. On a map with<br>
> central meridian of zero, I'd want to see half the polygon on the left<br>
side of the map and the other half on the right side.<br>
<br>
> CREATE TABLE example AS<br>
> SELECT ST_ConvexHull(<br>
> ST_Collect(ST_GeomFromText('MULTIPOINT(175 5, 175 30, -175 5, -175<br>
30)') ))::geography(Polygon, 4326) ;<br>
<br>
My guess is you'll have to cut your area into pieces. Still then its not<br>
that pretty when you try to rejoin.<br>
<br>
Sorry couldn't be more help,<br>
Regina<br>
<a href="http://www.postgis.us" target="_blank">http://www.postgis.us</a><br>
<br>
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Message: 3<br>
Date: Tue, 6 Sep 2011 22:21:48 -0700<br>
From: Paul Ramsey <<a href="mailto:pramsey@opengeo.org">pramsey@opengeo.org</a>><br>
Subject: Re: [postgis-users] Problem with convex hulls that cross the<br>
dateline<br>
To: PostGIS Users Discussion <<a href="mailto:postgis-users@postgis.refractions.net">postgis-users@postgis.refractions.net</a>><br>
Message-ID:<br>
<CACowWR0YVqT5_wasbLoscmCnqNFAqVT2ioA=<a href="mailto:yJQ6Uv_yX9Cv0A@mail.gmail.com">yJQ6Uv_yX9Cv0A@mail.gmail.com</a>><br>
Content-Type: text/plain; charset=ISO-8859-1<br>
<br>
On Tue, Sep 6, 2011 at 10:18 PM, Paragon Corporation <<a href="mailto:lr@pcorp.us">lr@pcorp.us</a>> wrote:<br>
> ?Sarah,<br>
> ?> ?I'm having trouble making convex hulls that cross the international<br>
> dateline. Imagine 4 points making a square with the international dateline<br>
> running down the middle--I would like the convex hull to be that square, but<br>
> instead I get a giant rectangle that goes across the entire planet. I've<br>
> done some reading, and it sounds like GEOS generally has a hard time dealing<br>
> with the dateline--is that accurate?<br>
> Yes GEOS? only deals with planar coordinates.? Geography is the only<br>
> geodetic aware type and there is no ConvexHull function for it.<br>
<br>
Although you could hack one up easily enough by copying the ideas in<br>
ST_Buffer(geography)<br>
<br>
P<br>
<br>
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<br>
Message: 4<br>
Date: Wed, 7 Sep 2011 13:43:52 +0800<br>
From: Ben Madin <<a href="mailto:lists@remoteinformation.com.au">lists@remoteinformation.com.au</a>><br>
Subject: Re: [postgis-users] Problem with convex hulls that cross the<br>
dateline<br>
To: PostGIS Users Discussion <<a href="mailto:postgis-users@postgis.refractions.net">postgis-users@postgis.refractions.net</a>><br>
Message-ID:<br>
<<a href="mailto:6AFE8509-2218-4CB2-956A-1269AB5CDB4B@remoteinformation.com.au">6AFE8509-2218-4CB2-956A-1269AB5CDB4B@remoteinformation.com.au</a>><br>
Content-Type: text/plain; charset=us-ascii<br>
<br>
Sarah,<br>
<br>
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...?<br>
<br>
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 - <a href="http://www.spatialreference.org/ref/epsg/3577" target="_blank">http://www.spatialreference.org/ref/epsg/3577</a>) and shift the lat and lon parameters...<br>
<br>
cheers<br>
<br>
Ben<br>
<br>
<br>
On 07/09/2011, at 1:13 AM, Sarah Berke wrote:<br>
<br>
> Hello,<br>
><br>
> 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!<br>
><br>
> 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.<br>
><br>
> CREATE TABLE example AS<br>
> SELECT ST_ConvexHull(<br>
> ST_Collect(ST_GeomFromText('MULTIPOINT(175 5, 175 30, -175 5, -175 30)') ))::geography(Polygon, 4326) ;<br>
><br>
><br>
> Thanks very much,<br>
> Sarah<br>
><br>
> _________________________________<br>
> Sarah K Berke<br>
> Postdoctoral Scholar<br>
> Department of the Geophysical Sciences<br>
> University of Chicago<br>
> 5734 S. Ellis Ave<br>
> Chicago, IL 60637<br>
> _______________________________________________<br>
> postgis-users mailing list<br>
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