[postgis-users] ST_Simplify and projections

[Sandro Santilli]

Hi Paolo,

On Tue, Jan 08, 2013 at 01:02:15PM +0100, Paolo Crosato wrote:
> Hello,
> 
> I'm trying to simplify a set of administrative boundaries that spans
> over the whole northern emishpere, boundaries are:
> 
> xMin,yMin -180,27.6378 : xMax,yMax 180,83.6274, CRS is 4326 WGS84.
> The whole dataset comprises 100k features and 54 million vertices,
> so it's quite big.
> 
> 
> Since I have to preserve the topology, I'm using topologies to do
> the simplification, as in this article: http://strk.keybit.net/blog/2012/04/13/simplifying-a-map-layer-using-postgis-topology/.
> The simplification process uses ST_simplify, and I know that using
> this function with non planar projections is not recommended. I
> still get good results with 4326, even at medium scales.
> However at large scales, like 1:17893297 or so (zoom level 5 to 1 on
> a tiled map system), there are visible artifacts. I've tried to
> simplify using lambert projection
> (EPSG:2154) on a single country, and the results are slightly better.
> 
> 
> I'm not an expert of geographical projections, so I'd like to know
> if there is some sort of planar projection supported by postgis,
> that spans over the whole northern emisphere.
> Visual distortion is not an issue, my idea is to project the whole
> set to this new CRS, simplify and reproject back the results to
> 4326. Simplifying single countries of the dataset using different
> projections is not feasible, since I have to simplify the whole set
> at once to preserve the topology.

Why not simplify single _edges_ of the topology using different
projections ? I mean, for each edge:
 - project to best projection for it
 - simplify
 - project back
Doing the above in the loop performed by my wrapper function (in the
blog post) should guard against topology errors.

SRID of "best projection" may currently be done using the internal
and undocumented _ST_BestSRID(geography) function. 
It may actually be interesting to expose an
ST_Simplify(geography, <meters>) to make this even simpler.

--strk;

 http://www.cartodb.com - Map, analyze and build applications with your data

                                       ~~ http://strk.keybit.net