# [postgis-users] ST_Buffer + grid problem

Ed Linde edolinde at gmail.com
Wed Mar 21 14:09:04 PDT 2012

```I am actually wondering how I can know the extents of this grid? So
basically I have some other
data set that has lat/long points. I will project them to the same as this
buffer/grid solution so that
from the points in that other data set I can just compute which grid cell
they belong to. So for
that I need to know what is the grid's top-left corner's lat/long right?
And then it should just be a
matter of calculating which grid cell a certain lat/long point should
belong to.

On Wed, Mar 21, 2012 at 9:53 PM, Ed Linde <edolinde at gmail.com> wrote:

> Thanks Pierre for the detailed information. I will give this a shot
> tomorrow and see how it goes! :)
>
>
> On Wed, Mar 21, 2012 at 8:46 PM, Pierre Racine <
> Pierre.Racine at sbf.ulaval.ca> wrote:
>
>> > I am not sure if this is possible, but I have computed (using
>> ST_Buffer) a sort of
>> > buffer around several LINESTRINGs. Now I would like to lay some sort of
>> grid of
>> > say 1m^2 cell size on top of this collection of geometries and then
>> compute the
>> > intersection... so in the end I would like to for example, know that
>> grid cell ID = 1
>> > intersects with buffers 1 and 10, grid cell 2 intersects with buffers
>> 7, 10 etc..
>> >
>> > Is there a simple way of doing this in postgis? Maybe someone could
>> point me to
>> > some documentation of how I can generate such a grid in postgis and
>> maybe
>> > then I can use just ST_Intersect once I have these two geometries?
>>
>> With the raster type you can now easily create a vector grid like this:
>>
>> CREATE TABLE vectorgrid AS
>> SELECT (gvxy).geom, ((gvxy).x - 1) * rwidth + (gvxy).y gridid
>> FROM (SELECT ST_PixelAsPolygons(rast) gvxy, ST_Width(rast) rwidth
>>             FROM (SELECT ST_AsRaster(ST_Extent(geom)::geometry, 1.0, 1.0)
>> rast
>>                          FROM yourbuffertable
>>                         ) foo1
>>            ) foo2;
>>
>> Make sure a spatial index exist on both tables:
>>
>> CREATE INDEX yourbuffertable_geom_idx  ON yourbuffertable USING gist
>>  (geom);
>> CREATE INDEX vectorgrid _geom_idx  ON vectorgrid USING gist  (geom);
>>
>> You can then perform a normal intersect query:
>>
>> CREATE TABLE interresult AS
>> SELECT b.bufferid, g.gridid, ST_Intersection(g.geom, b.geom) geom
>> FROM vectorgrid g, yourbuffertable b
>> WHERE ST_Intersects(g.geom, b.geom);
>>
>> Pierre
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at postgis.refractions.net
>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20120321/031728ba/attachment.html>
```