[postgis-users] ST_SnapToGrid details

[Sandro Santilli]

On Tue, Jun 25, 2013 at 11:48:03AM +0200, Egil Möller wrote:
> Hi!
> 
> I'm implementing a dynamic clustering of points in a map, where if you
> zoom out, any area with too many points in it has its points replaced by
> a big circle with the number of points it's covering written on it. When
> you zoom in, the big circles are gradually replaced with several smaller
> circles and finally with individual points.
> 
> On the server side this is implemented using ST_SnapToGrid followed by a
> group by (with count) over the table with the points. Then for each row
> where count < some number, a new select query is done to get the actual
> points in the area.
> 
> The problem is that I seem to get the are to select points from wrong.
> So, here's the question:
> 
>     Given that
> 
>     ST_SnapToGrid(ST_Point(lon, lat), sizeLon, sizeLat) =
>     ST_Point(resultLon, resultLat)
> 
>     for a given pair of values resultLon and resultLat, and given
>     sizeLon and sizeLat, what are the min and max values for lon and lat?

They should be :

 resultLong +/- sizeLon
 resultLat +/- sizeLat

> This does not seem to be documented anywhere in
> http://postgis.net/docs/manual-2.0/ST_SnapToGrid.html :(

Patch welcome (after confirmation:)

--strk;