Thanks for your feedback, Darrel! I hope, your query results will look fine.
Regards,
Birgit
Am 24.11.2015 um 14:00 schrieb Darrel Maddy:
Dear Birgit,
Very many thanks. The query is now running – so it is no longer giving
an error. I will not know for a few hours whether it is calculating
the necessary values but it seems progress is being made.
I have a book coming on SQL – I clearly need to do my homework more
thoroughly J
I will confirm when it stops!
Best wishes
Darrel
*From:*postgis-users [mailto:postgis-users-boun...@lists.osgeo.org]
*On Behalf Of *Birgit Laggner
*Sent:* 24 November 2015 12:19
*To:* postgis-users@lists.osgeo.org
*Subject:* Re: [postgis-users] Help with SQL query?
Hi Darrel,
my PostGIS version is too old for testing, but if I read the
documentation right, then your expression has to be SQL. And IF THEN
ELSE etc. is not SQL as far as I know - SQL has CASE WHEN.
So, I would assume, you would need to write your expression like this:
'CASE WHEN [rast2] > 0.0 THEN [rast1] ELSE NULL END'
I am curious if this helps with your error...
Regards,
Birgit
Am 24.11.2015 um 10:09 schrieb Darrel Maddy:
Dear Roxanne,
Many thanks. I did pickup that issue shortly after my last post
and it cured the first problem.
There is still an SQL issue with the expression however.
I tried ‘IF [rast2]>0.0 THEN [rast1] ELSE NULL ENDIF’
This produces an error at $2. I can remove that problem by doing this
‘IF ([rast2]>0.0) THEN [rast1] ELSE NULL ENDIF’
But then I get the error at THEN
Curiously ‘[rast2]+[rast1]’ works as intended so somehow the
conditional is being specified incorrectly.
Unfortunately there is nothing in the documentation which helps me
with this variant.
I will keep trying.
Darrel
*From:*postgis-users
[mailto:postgis-users-boun...@lists.osgeo.org] *On Behalf Of
*Roxanne Reid-Bennett
*Sent:* 24 November, 2015 12:46 AM
*To:* postgis-users@lists.osgeo.org
<mailto:postgis-users@lists.osgeo.org>
*Subject:* Re: [postgis-users] Help with SQL query?
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:postgis-users-boun...@lists.osgeo.org] *On Behalf Of
*Pierre Racine
*Sent:* 23 November 2015 20:30
*To:* PostGIS Users Discussion <postgis-users@lists.osgeo.org>
<mailto:postgis-users@lists.osgeo.org>
*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:postgis-users-boun...@lists.osgeo.org] *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:postgis-users-boun...@lists.osgeo.org] *On Behalf Of
*Darrel Maddy
*Sent:* 23 November 2015 16:30
*To:* PostGIS Users Discussion <postgis-users@lists.osgeo.org
<mailto:postgis-users@lists.osgeo.org>>
*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:postgis-users-boun...@lists.osgeo.org] *On Behalf Of
*Pierre Racine
*Sent:* 23 November 2015 16:17
*To:* PostGIS Users Discussion <postgis-users@lists.osgeo.org
<mailto:postgis-users@lists.osgeo.org>>
*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:postgis-users-boun...@lists.osgeo.org] *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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[At other schools] I think the most common fault in general is to teach
students how to pass exams instead of teaching them the science.
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