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<div class="moz-cite-prefix">On 02/10/2012 17:43, Lars Dalby wrote:<br>
</div>
<blockquote cite="mid:1349192616141-5005898.post@n6.nabble.com"
type="cite">
<pre wrap="">Micha Silver wrote
</pre>
<blockquote type="cite">
<pre wrap="">Here's the same query using db.select:
echo 'SELECT a_row AS GridRow, a_col AS GridCol, SUM(clip_area) As
"Water Area in Grid" FROM glwd_grid GROUP BY a_cat ORDER BY a_row, a_col
LIMIT 20;' | db.select
GridRow|GridCol|Water Area in Grid
9|43|11734990.811648
10|42|7600679.947002
10|43|20154750.823557
10|44|10095944.174832
10|46|18364623.744757
10|49|3054521.141835
11|41|250083.456892
11|42|9839134.964962
11|43|23519926.337785
11|44|11323614.28017
11|45|8426141.099425
11|46|12079532.459832
11|47|11135300.910967
11|49|571804.968053
11|50|1319664.199523
12|41|18036461.225015
12|42|15664405.025981
12|43|2471761.404269
12|45|2339714.549891
12|47|2890312.572311
</pre>
</blockquote>
<pre wrap="">
So I got this now, but now I struggle with getting these summed values set
in at the right locations in the vector grid (or ideally a raster, which is
the ultimate goal).
I guess the idea is to create a new vector from the sql selection and then
rasterize this new map, but I am affraid I will have to ask for a hint once
again...
</pre>
</blockquote>
<br>
Well this would be easy to do right within SQLite. After creating
the grid in GRASS you'll have an SQLite table called glwd_grid of
all areas of water surface (clipped to the grid) and another table
of the grid itself (I called it grid92 in the example). So, in
SQLite:<br>
<br>
ALTER TABLE grid92 ADD COLUMN water_surf_area DOUBLE PRECISION;<br>
<br>
Then use the above query as a subquery in an UPDATE Statement:<br>
<br>
UPDATE grid92 SET water_surf_area=(<br>
SELECT SUM(clip_area) <br>
FROM glwd_grid AS gg<br>
WHERE gg.a_cat=grid92.cat ) ;<br>
<br>
That should leave you with a vector polygon layer "grid92" including
a column "water_surf_area" which holds the area covered by water in
each grid cell.<br>
<br>
To convert that to a raster, using the water surface area would
require: extracting centroids, exporting the centroid attribute
table to ascii, and using that with r.in.xyz to recreate the raster.<br>
<br>
v.extract grid92 type=centroids out=grid92_centroids<br>
# GRASS keeps the attrib table linked to the centroids, <br>
# so when you extract centroids you also get the attribs<br>
v.out.ascii grid92_centroids out=grid92_centroids.txt
columns="cat,water_surf_area,...<other data>..."<br>
# Check the ascii file to see column positions<br>
<br>
Now set the resolution to exactly what you want for the final
raster:<br>
<br>
g.region -p res=92000,92000 n=... s=... e=... w=... <br>
<br>
Use the r.in.xyz -s option to examine the extent of the xyz point
data from the vector grid. Then<br>
<br>
r.in.xyz -s grid92_centroids.txt<br>
r.in.xyz in=grid92_centroids.txt out=grid92 z=<the column number
containing the water_surf_area data><br>
<br>
The above seems a bit convoluted. Maybe someone else can see a more
straight forward solution...<br>
<br>
Regards,<br>
Micha<br>
<br>
<br>
<blockquote cite="mid:1349192616141-5005898.post@n6.nabble.com"
type="cite">
<pre wrap="">
Thanks again
-Lars
--
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</pre>
</blockquote>
<br>
<br>
<pre class="moz-signature" cols="72">--
Micha Silver
052-3665918
</pre>
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