On 11/23/2015 12:41 PM, Darrel Maddy wrote:
Dear Pierre,
I was not looking for a total solution and I am grateful for the
suggestion.
Although it may not look like it, I did consult the documentation and
also I have Regina’s book beside me. Unfortunately, for me at least,
both documents assume some knowledge of SQL – which I do not have. I
am also trying to do this simultaneously with a large number of other
things that are new to me. I do not find the errors reported at all
informative and consider the query I am trying to perform to be
relatively trivial and hence I had hoped the structure of the query
might have been more intuitive. For others it may be.
FWIW - I don't play with rasters, but this appears to be a pure SQL
thing... add "as rast" like below and try again.
SELECT ST_MapAlgebra(deposition.rast, concentrated.rast, 'IF
concentrated > 6 THEN deposition ELSE NULL ENDIF ' )
as rast
FROM mymodel.deposition, mymodel.concentrated
WHERE ST_UpperleftX(mymodel.deposition.rast) =
ST_UpperleftX(mymodel.concentrated.rast) AND
ST_UpperleftY(mymodel.deposition.rast) =
ST_UpperleftY(mymodel.deposition.rast)
Roxanne
I will try not to bother you again.
Darrel
*From:*postgis-users [mailto:[email protected]]
*On Behalf Of *Pierre Racine
*Sent:* 23 November 2015 20:30
*To:* PostGIS Users Discussion <[email protected]>
*Subject:* Re: [postgis-users] Help with SQL query?
The expression has to stay as it was: 'IF [rast2] > threshold THEN
[rast1] ELSE NULL ENDIF '
Just replace the threshold value as you did.
Do not try to replace the [rast2] and [rast1]. They refer to the first
and second raster pixel values. Read the ST_Mapalgebra doc…
Don’t expect our suggestions to work blindly. I did not test this
query. I’m not in your context. I expect you read the doc about all
the mentioned functions and adjust for your specific context. I said
“your query should “look like” this”…
Pierre
*From:*postgis-users [mailto:[email protected]]
*On Behalf Of *Darrel Maddy
*Sent:* Monday, November 23, 2015 2:47 PM
*To:* PostGIS Users Discussion
*Subject:* Re: [postgis-users] Help with SQL query?
OK I spoke too soon.
I tried this:
SELECT (ST_SummaryStats(ST_Union(rast))).sum sum
FROM (SELECT ST_MapAlgebra(deposition.rast, concentrated.rast, 'IF
concentrated > 6 THEN deposition ELSE NULL ENDIF ' )
FROM mymodel.deposition, mymodel.concentrated
WHERE ST_UpperleftX(mymodel.deposition.rast) =
ST_UpperleftX(mymodel.concentrated.rast) AND
ST_UpperleftY(mymodel.deposition.rast) =
ST_UpperleftY(mymodel.deposition.rast) ) foo;
And all I get is rast does not exist.
I’m afraid the penny has not dropped yet L
Darrel
*From:*postgis-users [mailto:[email protected]]
*On Behalf Of *Darrel Maddy
*Sent:* 23 November 2015 16:30
*To:* PostGIS Users Discussion <[email protected]
<mailto:[email protected]>>
*Subject:* Re: [postgis-users] Help with SQL query?
Dear Pierre and Rasmus,
Many thanks for trying to help.
Rasmus: I am aware of GMT but I was looking for a solution in postgis
so that I can keep all of the data extraction in one place.
Pierre: That is exactly what I was looking for and very many thanks
for including the explanation. I am a little overwhelmed with the
number of functions offered in postgis. It is certainly a remarkable
tool. Watching the queries plough through my datasets is a pleasure –
albeit the results do not always please me J
Hopefully I can put this to work later tonight.
Best wishes
Darrel
*From:*postgis-users [mailto:[email protected]]
*On Behalf Of *Pierre Racine
*Sent:* 23 November 2015 16:17
*To:* PostGIS Users Discussion <[email protected]
<mailto:[email protected]>>
*Subject:* Re: [postgis-users] Help with SQL query?
Darrel,
1)Create a new raster selecting the right pixels with
ST_MapAlgebra(raster, raster)
2)Make sure only intersecting rasters are processed by using their
upper left corner X and Y coordinates (with ST_UpperLeftX() and
UpperLeftY())
3)Sum the selected pixels with ST_SummaryStats(rast)
All in all a global query should look like this:
SELECT (ST_SummaryStats(ST_Union(rast))).sum sum
FFOM (SELECT ST_MapAlgebra(tableA.rast, tableB.rast, 'IF [rast2] >
threshold THEN [rast1] ELSE NULLENDIF ' )
FROM tableA, tableB
WHERE ST_UpperleftX(tableA.rast) = ST_UpperleftX(tableB.rast) AND
ST_UpperleftY(tableA.rast) =
ST_UpperleftY(tableB.rast) AND
maybe some other condition here if you get
time series e.g. tableA.year = tableB.year AND tableA.month =
tableB.month) foo
If you have millions of tile you could create indexes on
ST_UpperleftX(tableA.rast), ST_UpperleftX(tableB.rast),
ST_UpperleftY(tableA.rast) and ST_UpperleftY(tableB.rast) to make the
query faster.
You could also just use WHERE ST_Intersects(tableA.rast, tableB.rast)
instead…
Pierre
*From:*postgis-users [mailto:[email protected]]
*On Behalf Of *Darrel Maddy
*Sent:* Monday, November 23, 2015 7:20 AM
*To:* PostGIS Users Discussion
*Subject:* [postgis-users] Help with SQL query?
Dear all,
As you know I am relatively new to postgis and SQL and therefore I
have much to learn. However, I am facing a paper deadline and need to
do some quick analysis of the data I have and I am struggling to
figure out how best to pursue what I need to do.
I have a significant number of rasters which have double precision
values. Without going into detail about what the rasters represent, I
need to extract and sum values from one set of rasters in say table A
based upon values in another set of rasters in say table B where the
pixel value in the raster from Table B exceeds a threshold. Both
tables are the same size (rasters are tiled) but I also need to figure
out how I make sure the correct rasters are compared. They have
filenames like this rastervariable_10.tif, rastervariable_100.tif ,
presumably I need to use a logical expression to strip the numerical
value (in this case this represents the year) and then order on that
basis?
I can do this in QGIS one at a time but that is a little clumsy and
rather time consuming.
If someone can just point me in the right direction I am sure I can
figure out the rest for myself.
Apologies once more for asking what is probably a rather trivial
question and yet again demonstrating my ignorance.
Many thanks
Darrel
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