[postgis-users] Raster pixel value

[Andreas Forø Tollefsen]

Hi Tom,

I tried both functions. The St_AreaWeightedSummaryStats() works great, but
it takes a lot of time to complete.

However, the ST_Clip() i cannot manage to get working. Seems like something
is broken. Any idea what can cause this error?
I installed the latest ST_SummaryStatsAgg() and ST_Clip() from
http://trac.osgeo.org/postgis/browser/trunk/raster/scripts/plpgsql

The query:
SELECT foo2.gid,
(ss).count,
      (ss).sum,
      (ss).mean,
      (ss).min,
      (ss).max
FROM
(SELECT foo.gid, ST_SummaryStatsAgg(gv) ss
     FROM (SELECT p.gid, ST_Clip(r.rast, p.cell) gv
           FROM access r, priogrid_land p
           WHERE ST_Intersects(r.rast, p.cell)
          ) foo
     GROUP BY foo.gid
    ) foo2

Results in:

ERROR:  function st_minpossiblevalue(text) does not exist
LINE 1: ...esce(nodata, ST_BandNodataValue(rast, bandstart), ST_MinPoss...
                                                             ^
HINT:  No function matches the given name and argument types. You might
need to add explicit type casts.
QUERY:  SELECT coalesce(nodata, ST_BandNodataValue(rast, bandstart),
ST_MinPossibleValue(newpixtype))
CONTEXT:  PL/pgSQL function "st_clip" line 31 at assignment

********** Error **********

ERROR: function st_minpossiblevalue(text) does not exist
SQL state: 42883
Hint: No function matches the given name and argument types. You might need
to add explicit type casts.
Context: PL/pgSQL function "st_clip" line 31 at assignment



2011/12/6 Tom van Tilburg <tom.van.tilburg at gmail.com>

>  Andreas,
>
> I didn't have time to reproduce your problem yet. Did you have any succes
> by yourself on this issue?
> Could it have something to do with counting the non-data values as value?
> This is what I experience with a similar function (ST_Clip) that
> consequently gave me the value '0' instead of the nodata value. The result
> of that is that the mean was often lower than expected.
>
> Perhaps you could rewrite your previous example to something with
> auto-generated values in the script. That saves time in reproducing.
>
> Cheers,
>  Tom
>
>
>
> On 25-11-2011 18:11, Andreas Forø Tollefsen wrote:
>
> Update:
>
>  I think my suspicion is correct. If I do a ST_Summarystats().sum and
> divide this on 36 my MAX value will be 1.
> Hence, I think the number of values counted and the number of observations
> counted is not equal.
>
>  New query:
>  DROP TABLE IF EXISTS mountain_phil_cell;
>
>  SELECT
> a.gid As gid,
> (ST_SummaryStats((ST_Union(ST_MapAlgebraExpr(ST_AsRaster(a.cell, b.rast,
> '32BF'), b.rast, 'rast2', '32BF','INTERSECTION','0','0',0))).rast,
> false)).sum / 36 As avgmnt
> INTO mountain_phil_cell
> FROM
> priogrid_land a LEFT JOIN
> mountain_phil b
>     ON ST_Intersects(a.cell, b.rast)
> GROUP BY a.gid
> ORDER BY a.gid;
>
>
> 2011/11/25 Andreas Forø Tollefsen <andreasft at gmail.com>
>
>> A small note regarding this issue.
>>
>>  My problem is that I never get a mean value of 1 even if all pixels
>> inside the geometry is one.
>>
>>  Could this be because: 6x6 pixels goes into one polygon when visually
>> controlling. If each pixel has the value 1, then this will be calculated as
>> 36 / 36 = 1. However, if it calculates the sum to be 36 and divide by a
>> number higher than 36 pixels, then the result will always be below 1.
>> What i am thinking is that while it sums up the pixel values correctly,
>> it does not count only the 36 pixels, but also neighboring pixels.
>> Therefore: 1+1+1+1...n36 / Number of pixels higher than 36 will always lead
>> to a number lower than 1.
>>
>>  Anyone who knows the functions well could probably answer this.
>>
>>  Best regards,
>>  Andreas
>>
>>
>> 2011/11/25 Andreas Forø Tollefsen <andreasft at gmail.com>
>>
>>> Could this have to do with the tiling of the raster?
>>> I will try to run the same query with a untiled mountain raster to see
>>> if that changes anything.
>>>
>>>  Btw. When loading a tiled postgis raster into qgis it shows up with
>>> many artifacts and no data areas. The same raster untiled does not show up
>>> the same way.
>>> Qgis bug?
>>>
>>>  Andreas
>>>
>>> 2011/11/25 Andreas Forø Tollefsen <andreasft at gmail.com>
>>>
>>>>  Hi,
>>>>
>>>>  Thanks for all of the suggestions. I will do some more testing.
>>>> However, as for suggestion 1 i think the pixel size should be the same as
>>>> the original raster or am I wrong?
>>>>
>>>>  Both the mean_mnt_bin raster and the priogrid_land shapefile can be
>>>> downloaded as zip (2 mb) here:
>>>> http://gisintersect.com/mean_mnt_bin.zip
>>>> http://gisintersect.com/priogrid_land.zip
>>>>
>>>>  Any help on getting the correct values would be very much appreciated.
>>>>
>>>>  My query:
>>>>  DROP TABLE IF EXISTS mountain_cell;
>>>>
>>>>  SELECT
>>>> a.gid As id,
>>>> (ST_SummaryStats((ST_Union(ST_MapAlgebraExpr(ST_AsRaster(a.cell,
>>>> b.rast, '32BF'), b.rast, 'rast2', '32BF','INTERSECTION','0','0',0))).rast,
>>>> false)).mean As avgmnt
>>>> INTO mountain_cell
>>>> FROM
>>>> priogrid_land a LEFT JOIN
>>>> mountain b
>>>>     ON ST_Intersects(a.cell, b.rast)
>>>> GROUP BY a.gid
>>>> ORDER BY a.gid;
>>>>
>>>
>>>
>>
>
>
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