[postgis-users] Re: Polygon and north pole

[Jørn Vegard Røsnes]

On Monday 16 July 2007 15:45, Paul Ramsey wrote:
> The IsValid function does not exist unless you have JTS/GEOS support.
> Your geometry has nothing to do with this error message.
>
Thanks.

> On the topic of your geometry, you can insert it, but in a planar
> interpretation of lon/lat, which is what we have, it will not be
> handled "right" in every case, and if it self-intersects in planar
> space (a common situation for pole-covering things) it will be
> "invalid". It will still be storable, though.
>

I think many polygons will be invalid then, and I assume not queryable.
I will try to explain the "challenge" and suggest a workaround

I have some satellite products (Terra/Aqua MODIS) where the product (polygon) 
often cross the north pole. 
I want to do spatial queries on these products, the query is a lat/lon 
polygon.

Possible workaround:
The product polygons are stored in PostGIS as geometries in (polar) 
stereographic projection, by utilizing the transform function and for example 
SRID 32661 as input parameter (select srid,proj4text from spatial_ref_sys 
where proj4text like '%stere%';).

If I utilize the PostGIS transform function to transform the search polygon to 
a (polar) stereographic geometry (SRID 32661) and execute spatial queries to 
match the product geometries in (polar) stereographic.

Will PostGIS then use the "stereographic space", not the planar/Euclidean 
space?

I may not have used the proper definitions in this e-mail, but hopefully you 
understand the issue.

cheers
Jørn Vegard

> P
>
> On 16-Jul-07, at 4:30 AM, Jørn Vegard Røsnes wrote:
> > Hi,
> >
> > is it possible to insert polygons into PostGIS where the polygon
> > "cross" the
> > north pole?
> >
> > Example:
> >
> > select isvalid('POLYGON((152.991022 78.938867,79.710092
> > 68.321799,-50.514656
> > 62.226884,-9.980464 56.376708, 152.991022 78.938867))');
> > ERROR:  isvalid:: operation not implemented - compile PostGIS with
> > JTS or GEOS
> > support
> >
> > Will it help to compile PostGIS with GEOS/JTS support?
> >
> > cheers
> > Jørn Vegard Røsnes
> >
> >
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