[geos-devel] polar geometry spanning 0-360 lon
Mike Toews
mwtoews at gmail.com
Tue Mar 11 14:53:58 PDT 2014
Hi David,
GEOS (and JTS) use Cartesian space, so concepts of latitude and
longitude are irrelevant. The choice of projection and precision are
important for spatial operations, but these need to be handled
externally using libraries like PROJ.4, or other logic for handling
latitude and longitude.
The two valid polygons look like skinny lines in a Cartesian
representation of EPSG:4326, but look more like squares when
transformed to EPSG:3031. It's not clear what you were expecting, so I
cant tell what is considered a "fail". The intersection results are
the same for JTS, and does not seem like a bug to me.
Usually the best approach is to do spatial operations with GEOS in a
projected Cartesian space (on either pole, for your case). But only do
this if it is within the range of the projection's limits. This is
further limited by the size of the geometry, as sometimes they don't
fit within the projection nicely. But these are general GIS concerns
that are out of scope for GEOS to consider.
-Mike
On 12 March 2014 08:29, David Shean <dshean at gmail.com> wrote:
> Hi list,
> I've run into this a few times now, and I'm hoping someone can point me in the right direction. I primarily use GEOS through the ogr API to work with polar data in either EPSG:3031 or EPSG:3413. When I work with smaller geometries in these projections, I always convert to WGS84 before performing any operations, and this works well. However, even this seems to fail for large polygons that span 0-360 around the pole:
>
> g1 = POLYGON ((-45.0 -54.673528789190144,45.00716107726717 -54.669377943743747,135.0 -54.66522770972778,-135.00716107726717 -54.669377943743747,-45.0 -54.673528789190144))
> g2 = POLYGON ((-45.0 -54.673528789190144,45.00255773311347 -54.672046393296853,135.0 -54.670564075397799,-135.002557733113463 -54.672046393296853,-45.0 -54.673528789190144))
> g1.Intersection(g2)
> POLYGON ((3.650402839379899 -54.671285186562592,-45.0 -54.673528789190144,-83.299396823994513 -54.67176254220486,3.650402839379899 -54.671285186562592))
>
> For reference, the EPSG:3031 geom is POLYGON ((-3333500 3333500,3333500 3333500,3333500 -3333500,-3333500 -3333500,-3333500 3333500)).
>
> This looks like a bug to me, but maybe there is a way to specify that the polygon should include minlat -90? Thanks.
> -David
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