Generating areas from point datasets with PostGIS

[pcreso]

Hi,

Any comments or advice appreciated...

I have a table (155m points) of circumpolar depth points.

From this I want to use POstGIS to generate polygons based on depths.

There are about 700,000 points in the Ross Sea region, each has a depth (or altitude) associated with it. The points are 1/2nm apart (GEBCO 2008 data)

I figure the following approach will work to generate reasonable polygons for areas in the Ross Sea shallower than 550m:

select astext(geomunion(envelope(buffer(geom,1.0/230.0))))
                from srtm_ant
                where depth <0
                  and depth >-500
                  and geom && setsrid(geometryfromtext('POLYGON((155 -80,155 -60,200 -60,200 -80,155 -80))'),4326);


The && WKT coords define the Ross Sea region, 1/2nm is 1/120 degree so a buffer of 1/230 degree should ensure the envelopes of the buffered points all overlap.


Does anyone have any suggestions/advice regarding this?

Better or faster approaches, etc.


Thanks,

  Brent Wood
_______________________________________________
postgis-users mailing list
postgis-users@...
http://postgis.refractions.net/mailman/listinfo/postgis-users

[Nicklas Avén]

I am not sure I understand what you want to do. Do you want one sqare-polygon for each point?

 

Now you are unioning them all together to one big polygon....

 

anyway

instead of :

envelope(buffer(geom,1.0/230.0))

I guess it's more effective to use:

st_expand(geom, 1.0/230.0)

 

http://www.postgis.org/documentation/manual-svn/ST_Expand.html

 

 

/Nicklas

 

 

 

2009-06-19 pcreso@... wrote:


>Hi,
>
>Any comments or advice appreciated...
>
>I have a table (155m points) of circumpolar depth points.
>
>From this I want to use POstGIS to generate polygons based on depths.
>
>There are about 700,000 points in the Ross Sea region, each has a depth (or altitude) associated with it. The points are 1/2nm apart (GEBCO 2008 data)
>
>I figure the following approach will work to generate reasonable polygons for areas in the Ross Sea shallower than 550m:
>
>select astext(geomunion(envelope(buffer(geom,1.0/230.0))))
> from srtm_ant
> where depth <0
> and depth >-500
> and geom && setsrid(geometryfromtext('POLYGON((155 -80,155 -60,200 -60,200 -80,155 -80))'),4326);
>
>
>The && WKT coords define the Ross Sea region, 1/2nm is 1/120 degree so a buffer of 1/230 degree should ensure the envelopes of the buffered points all overlap.
>
>
>Does anyone have any suggestions/advice regarding this?
>
>Better or faster approaches, etc.
>
>
>Thanks,
>
> Brent Wood
>_______________________________________________
>postgis-users mailing list
>postgis-users@...
>http://postgis.refractions.net/mailman/listinfo/postgis-users
>
>

_______________________________________________
postgis-users mailing list
postgis-users@...
http://postgis.refractions.net/mailman/listinfo/postgis-users