[postgis-users] Re: "Summarizing" a collection of geometries

[Reid Priedhorsky]

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