[postgis-users] ERROR: ptarray_area_spheroid: cannot handle ptarray that crosses equator
Ben Madin
lists at remoteinformation.com.au
Sun Jul 24 15:11:39 PDT 2011
G'day Sarah
I'm afraid I'm no expert on geography types, but yes, in some cases st_area can process invalid geometries (like you have described), but they are after all invalid geometries, so it returns 0 . For example, looking at your second example :
=# SELECT ST_AREA(the_geom), ST_ISVALID(the_geom), ST_ISVALIDREASON(the_geom), ST_SUMMARY(the_geom) FROM (SELECT ST_GEOMFROMTEXT('POLYGON((15 -5, 16 0, 15 0, 16 -2, 15 -5))') the_geom) as foo;
NOTICE: Self-intersection at or near point 15.7143 -1.42857
st_area | st_isvalid | st_isvalidreason | st_summary
---------+------------+-------------------------------------------------------+-------------------------
0 | f | Self-intersection[15.7142857142857 -1.42857142857143] | +
| | | Polygon[B] with 1 rings+
| | | ring 0 has 5 points +
| | |
(1 row)
it is not because one side rules is the inverse of the other that the area is 0, it just returns 0 not an error. My guess is that the invalid geography is causing an uncaught error, but there is no st_isvalid(geography), hence using it on geometry type. This would be my approach to finding the invalid geometries. How you repair them...? I guess it would depend on how they were originally collected, how much each one cost to collect or what they meant.
cheers
Ben
On 25/07/2011, at 5:27 AM, Sarah Berke wrote:
> I've figured out a little more detail about my ST_Area problem, perhaps this might help someone give me some advice for troubleshooting. To provide a little more background, I've generated several thousand polygons using ST_ConvexHull from collections of points (I've tried both ST_ConvexHull(ST_Collect(points)) and ST_ConvexHull(ST_Union(points)) ) and when I try to calcuate their areas I get this error:
>
> ERROR: ptarray_area_spheroid: cannot handle ptarray that crosses equator
> CONTEXT: SQL function "st_area" statement 1
>
> The problem seems to be that ST_Area cannot handle some polygons with internal crosses (i.e. bowtie-shaped) that share a border with the equator, even though it can handle them elsewhere. For example:
>
> SELECT ST_AREA(ST_GEOGFROMTEXT('POLYGON((15 -5, 15 5, 16 0, 16 -5, 15 -5))')) ;
> works just fine--this is just a simple polygon that crosses the equator. By the same token, a simple polygon that borders the equator works:
>
> SELECT ST_AREA(ST_GEOGFROMTEXT('POLYGON((15 -5, 15 0, 16 0, 16 -5, 15 -5))')) ;
>
> Switching the 2nd and 3rd coordinates, however, makes a bowtie-shaped polygon which is valid, but it shares a border with the equator and ST_Area cannot take it:
>
> SELECT ST_AREA(ST_GEOGFROMTEXT('POLYGON((15 -5, 16 0, 15 0, 16 -5, 15 -5))')) ;
> ERROR: ptarray_area_spheroid: cannot handle ptarray that crosses equator
> CONTEXT: SQL function "st_area" statement 1
> The same kind of formation works fine, however, if it does not share a border with the equator :
>
> SELECT ST_AREA(ST_GEOGFROMTEXT('POLYGON((15 -5, 16 1, 15 0, 16 -5, 15 -5))')) ;
> SELECT ST_AREA(ST_GEOGFROMTEXT('POLYGON((15 -4, 16 1, 15 1, 16 -4, 15 -4))')) ;
>
>
> Again, here are my system details:
> PostgreSQL 8.4.8 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real
> (Ubuntu 4.4.3-4ubuntu5) 4.4.3, 64-bit
> POSTGIS="1.5.2" GEOS="3.3.0-CAPI-1.7.0" PROJ="Rel. 4.7.1, 23 September
> 2009" LIBXML="2.7.6" USE_STATS
>
> I assume that one or more of the polygons I made with ConvexHull is a bowtie-shape sharing a border with the equator, but why would ST_ConvexHull generate a shape like that? How can I avoid this problem? Short of visually examining the map of every one of these several thousand polygons, how do I even figure out which one is the culprit? Is there a better way to go from points or multipoints to polygons?
>
> Any insight would be really great! Thanks!
>
> _________________________________
> Sarah K Berke
> Postdoctoral Scholar
> Department of the Geophysical Sciences
> University of Chicago
> 5734 S. Ellis Ave
> Chicago, IL 60637
>
>
> Date: Fri, 22 Jul 2011 14:39:52 -0500
> From: Sarah Berke <skberke at uchicago.edu>
> Subject: [postgis-users] ERROR: ptarray_area_spheroid: cannot handle
> ptarraythat crosses equator
> To: postgis-users at postgis.refractions.net
> Message-ID:
> <CAJUPZDEPTyKHb4LoG1szea0mSpRGikpFHM70Fh_AtBBuxSoYJA at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Hello,
>
> I found a short thread on this subject from February 2011--I am now having
> the same problem that Marcello had, and I wonder if anyone has found a fix
> or workaround for this problem? Using the same example code that Marcello
> provided:
>
> SELECT 161109 polygon ,ST_AREA(ST_GEOGFROMTEXT('POLYGON((15.8327132977611
> 0.25,16 0.25,16 0,15.8566682147637 0,15.8566112518311
> 0.00356000009924173,15.8485898971558 0.124770000576973,15.8327132977611
> 0.25))')) gsareageo;
>
> I get
>
> ERROR: ptarray_area_spheroid: cannot handle ptarray that crosses equator
> CONTEXT: SQL function "st_area" statement 1
>
> I am using:
>
> PostgreSQL 8.4.8 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real
> (Ubuntu 4.4.3-4ubuntu5) 4.4.3, 64-bit
> POSTGIS="1.5.2" GEOS="3.3.0-CAPI-1.7.0" PROJ="Rel. 4.7.1, 23 September
> 2009" LIBXML="2.7.6" USE_STATS
>
> >From the earlier thread, it sounds as if this version of postgis should run
> the code just fine? I saw that Marcello was able to work around the problem
> by changing 0 lats to being very near-0; but this of course affected the
> calculated areas. Can anyone guess why this is a problem, or has anyone
> figured out a better solution?
>
> Thanks very much!
> _________________________________
> Sarah K Berke
> Postdoctoral Scholar
> Department of the Geophysical Sciences
> University of Chicago
> 5734 S. Ellis Ave
> Chicago, IL 60637
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