[postgis-users] ST_Simplify and projections

[Paolo Crosato]

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.


Thanks for any feedback.


Regards,


Paolo

-- 
Paolo Crosato

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