[postgis-users] Create polygon from buffered points

[Dane Springmeyer]

John,

 From your description it seems that the hotspot points represent  
individual fires rather than locations around the periphery of  
hotspots. Just in case the latter is true it seems like a potential  
route would be to dissolve/union all points that relate to a single  
fire, then calculate the convex hull of these points, then calculate  
the area of that hull.

Here's the SQL:

SELECT ST_Area(St_ConvexHull(ST_Union(the_geom))) FROM hpt_test GROUP  
BY fire_group


If my sense of the former is what you are after, your SQL would look  
more like:

SELECT ST_Area(ST_Transform(ST_Union(ST_buffer(the_geom, 20000)), 
102001)) FROM hpt_test GROUP BY some_unique_hotspot_id

Where 102001 is the ESRI epsg code for Canadian Albers, although this  
projection is not in the spatial_ref_sys table of Postgis by default.  
You'll have to add it in. You can get the SQL insertion text from  
here: http://spatialreference.org/ref/epsg/102001/

That will give you a table listing of the areas of all the buffered  
polygons that overlap.


Cheers,

Dane





On Nov 14, 2007, at 3:11 PM, Little, John wrote:

> Hi there,
>
> I am new to this list (and to postgis). I'd really appreciate some  
> help with this problem (in my attempt to transfer this existing  
> process from ESRI to PostGIS).
>
> I have a table containing points (hpt_test) representing wildfire  
> hotspot locations, and I would like to create fire perimeter  
> estimates (polygons) using these point locations -- here are the  
> steps I intend to follow:
>
> 1) Buffer all points in table hpt_test (out 2000m) to produce  
> polygons (circles) -- to be inserted into table hpt_buf_test,
>
> 2) Dissolve where these circles overlap to create single buffer  
> polygons.
>
> 3) Perform an inside buffer (in -1300m) on the result polygon  
> produced in step 2 -- this will help correct polygon shape.
>
> 4) Reproject (transform) to equal area projection (Albers-Canada)  
> to calculate area
>
> The desired result will be a table containing rough fire perimeter  
> estimates (fairly accurate on large size wildfires).
>
> Here is what I've done to prepare the buffer polygon table :
>
>
> CREATE TABLE cwfis_datamart.hpt_buf_test ( ID int4, NAME varchar 
> (20) );
>
> SELECT AddGeometryColumn('cwfis_datamart','hpt_buf_test','the_geom', 
> 42304,'MULTIPOLYGON',2);
>
>
> Now that I've got the table set up, I attempt to add buffer  
> polygons to the table (step 1 above):
>
>
> INSERT INTO cwfis_datamart.hpt_buf_test (ID, NAME, the_geom)
> VALUES (1,'First Geometry',(SELECT buffer(the_geom,2000)
> FROM cwfis_datamart.hpt_test));
>
> But this (of course) does not work... and I have scoured the  
> archives for clues but without success:( Please help me get started  
> on this.
>
> I am aware that buffer is "a very expensive" process, but I can't  
> think of an alternative. My test point table is very small, but in  
> reality this would be a fairly large table, so I will likely need  
> to break this up by region… If anyone can help me out with the sql,  
> point me to helpful resources, or suggest a better alternative,  
> please advise :)
>
> Thanks!
>
> John Little
> http://cwfis.cfs.nrcan.gc.ca/
>
>
>
> _______________________________________________
> 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/20071114/64f2db9a/attachment.html>