[postgis-users] Raster cell size and wrong mean value calculation problem

[Simon Appelt]

Hi Bborie,

thanks for your fast answer! Wow, i really missed to read that :-(
But my second query ,which i posted below, provides the same Mean-Value 
result as the first. If i understand you correct, the st_clip command 
should end up with a different result .

Well i'll do some further testing tomorrow and keep the list up to date.

Simon


Am 23.01.2013 17:05, schrieb Bborie Park:
> By the looks of the docs for ST_Intersection, what you're getting is
> correct.  The second paragraph indicates that the raster gets
> vectorized using ST_DumpAsPolygon() which does merge neighboring cells
> with the same value in one larger geometry.
>
> http://postgis.net/docs/manual-2.0/RT_ST_Intersection.html
>
> The query using ST_Clip is more appropriate.
>
> -bborie
>
> On Wed, Jan 23, 2013 at 6:33 AM,  <fLaNsch at gmx.de> wrote:
>> Hi Postgis User-list,
>>
>> i've downloaded and migrated some srtm3 rasterdata into my PostgreSQL/Postgis 2.0 Database successfully. Now i would like to calculate the mean elevation value of a given polygon area by intersecting my polygon layer with the raster grid.
>>
>> Using a SQL-Statement, which was shown by Pierre Racine at the FOSS4G 2011:
>>
>> SELECT polyID,
>> (ST_Intersection(geom, rast)).geom poly,
>> (ST_Intersection(geom, rast)).val elevation
>> FROM polygon, srtm WHERE ST_Intersects(geom, rast);
>>
>>
>> What i expected the polygon-result geometry would look like:
>>   _ _ _ _
>> |_|_|_|_|
>> |_|_|_|_|
>> |_|_|_|_|
>> |_|_|_|_|
>>
>> What the result actually looks like:
>>   _ _ _ _
>> |___|_  |
>> |_  |_| |
>> |_|___|_|
>> |_|_|_|_|
>>
>>
>> The raster cells with the same elevation value got merged, why?
>> The mean value which based on the actual result cannot be correct. Same result different SQL-Statement without selecting the geometry:
>>
>> Select polyid,(stats).mean from (Select  a.polyid,st_summarystats(st_clip(b.rast,1,a.geom))as stats from polygon a, srtm b where ST_intersects(a.geom,b.rast )
>>
>>
>> I'm very grateful for any help!
>>
>> Best regards,
>> Simon
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