[postgis-users] Grid of points inside Polygon

[Rémi Cura]

Hey,
the whole point on using a sgbds like postgis is using index.

If you have one line you don't use indexes...

So in short, don't make one polygon with a buffer of all the road, but a
table with a line for the buffer for every road, then do you computation to
create grid of points inside of polygons, then union the result of points!

And it s always a bad idea to run a function on big data when you have not
tested it fully (including scaling behavior) on small data.


Cheers
Rémi-C


2013/11/11 James David Smith <james.david.smith at gmail.com>

> Hi all,
>
> Would appreciate some advice on the best way to accomplish this please.
>
> Our situation is that we have a single polygon which has been created
> by buffering all of the major roads in the UK. Projection is OSGB36
> (27700). Obviously it's quite a big polygon.
>
> -->  SELECT st_area(geom) FROM roadbufferunion;
>      st_area
> ------------------
>  77228753220.8271
>
> What we now want to do is create a regular grid of 20 metre x 20 metre
> points instead the polygon area. So we wrote this function (based on
> some googling, apologies for not being able to recall the exact person
> who originally wrote it):
>
> CREATE OR REPLACE FUNCTION makegrid(geometry, integer, integer)
> RETURNS geometry AS
> 'SELECT ST_Collect(st_setsrid(ST_POINT(x,y),$3)) FROM
>   generate_series(53320::int, 667380::int,$2) as x
>   ,generate_series(7780::int, 1226580::int,$2) as y
> where st_intersects($1,st_setsrid(ST_POINT(x,y),$3))'
> LANGUAGE sql
>
> and we then run this by doing the following:
>
> SELECT st_x((ST_Dump(makegrid(geom, 20, 27700))).geom) as x,
> st_y((ST_Dump(makegrid(geom, 20, 27700))).geom) as y INTO grid_points
> from roadbufferunion;
>
> However after over 2 days of the query running on a pretty powerful
> linux cluster, we still have no result.  I'm not sure if it is
> actually running or not to be honest.
>
> Does the query look right?
> Any ideas how we can make it run quicker?
>
> Thanks
>
> James
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