On Tuesday 22 April 2008, Reid Priedhorsky wrote: > Reid Priedhorsky wrote: > > Dear list, > > > > I would like to compute a "summary" of a collection of geometries > > (points, linestrings, polygons, or any combination). > > > > Here is the use case. I would like to summarize the geometry collection > > and present this summary visually so users working at a small scale can > > see, "ah, if I zoom in here, I can see more details about this geometry > > collection". > > > > The simple solution would be to compute a convex hull or bounding box, > > but this fails if the members of the collection are far apart (users > > could zoom into the middle and see nothing). In this case the summary > > would need to be a multipolygon. But the trick is figuring out when to > > split summary polygons into multipolygons. > > > > Has this been done? Do any of you have suggestions or references? > > > > It is very preferable to do this entirely in the database rather than in > > external code. Embedded procedures are fine. > > > > As you can see I am not entirely sure of my terminology, so any > > suggestions there would be welcome as well. > > Just to clarify, this is something I need to do now -- I don't want to > wait until a really slick way is implemented. > > Here's one idea: > > 1. Use ST_Buffer() to compute polygon buffers around members of the > collection. Question: ST_Buffer() docs say it can't handle > GeometryCollections. What about e.g. MultiPoint or MultiPolygon? > > 2. Take the ST_Union() of the result of #1, producing a MultiPolygon. > > Does seem like a reasonable approach? > > Thanks, > > Reid
Almost forgot: another simple approach: export your points as ASCII, something like: x|y|z|value import into GRASS with r.in.xyz: http://grass.osgeo.org/grass63/manuals/html63_user/r.in.xyz.html adjusting the region resolution will give you control over the final grid size upon which the summary stats are computed. I suppost the snaptogrid() in postgis might also do the trick. cheers, Dylan -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users