[postgis-users] ST_Value from Polygon

Thu Feb 24 06:13:22 PST 2011

Andreas,
Sorry should have recognized what you're doing.  The intersection returns a
polygon which is a union of the clipped raster pixel squares.  So you need
to use Sum of area instead and then divide by the area of a pixel to get the
equivalent of your count.

So 

SELECT gid, SUM(ST_Area((foo.geomval).geom))/ [put your pixel area size
here]  as ct
FROM (SELECT globshort.rid, priogrid_land.cell, priogrid_land.gid,
ST_Intersection(globshort.rast, priogrid_land.cell) AS geomval FROM
globshort, priogrid_land) AS foo
WHERE gid >= 139358 AND gid <= 139365
GROUP BY gid
ORDER BY gid

  _____  

From: Andreas Forø Tollefsen [mailto:andreasft at gmail.com] 
Sent: Thursday, February 24, 2011 8:33 AM
To: PostGIS Users Discussion
Cc: Paragon Corporation
Subject: Re: [postgis-users] ST_Value from Polygon

I am a bit unsure whether my results are actually correct. According to a
total count using the below query, I get very different results between the
cells. 
Since the raster does actually cover the whole vector cell, i would assume
that the count should be similar in all cells. Meaning, the pixel count
should be the same.
What i get is different, and it seems that the query is not providing me
with the number of pixels within the grid cell.
Any idea why this is so different?

SELECT gid, count((foo.geomval).val) as ct
FROM (SELECT globshort.rid, priogrid_land.cell, priogrid_land.gid,
ST_Intersection(globshort.rast, priogrid_land.cell) AS geomval FROM
globshort, priogrid_land) AS foo
WHERE gid >= 139358 AND gid <= 139365
GROUP BY gid
ORDER BY gid

Result:
139358;632
139359;1030
139360;912
139361;731
139362;760
139363;1230
139364;1314
139365;1014

The attached image shows the raster pixels within one cell.

2011/2/24 Andreas Forø Tollefsen <andreasft at gmail.com>

Thanks!  
That solved it.

This will probably take a lot of time. I have 259200 polygons measuring 0.5
x 0.5 decimal degrees while the raster dataset is of global cover and has a
pixelsize of 0.00277777777777778x0.00277777777777778. 

Andreas

2011/2/23 Paragon Corporation <lr at pcorp.us>

Andrea,

Try 

SELECT DISTINCT ON(gid) gid, (foo.geomval).val, COUNT((foo.geomval).val) AS
ct

FROM (SELECT globshort.rid, priogrid_land.cell, priogrid_land.gid,
ST_Intersection(globshort.rast, priogrid_land.cell) AS geomval FROM
globshort, priogrid_land) AS foo
WHERE gid > 151000 AND gid < 151010
GROUP BY gid, (foo.geomval).val
ORDER BY gid, ct DESC

  _____  

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Andreas
Forø Tollefsen

Sent: Wednesday, February 23, 2011 4:05 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] ST_Value from Polygon

Hi. Thanks Regina and Leo, 
I have been testing the raster and geom intersection a bit. I guess what i
need is to use the ST_Intersection together with a max(count) function. 
So my result will be the rastervalue with the highest count within each of
the grid cells.
However, as far as i know, there is now Max(COUNT) function in postgresql.

Any idea how i can modify the below query to only return the rastervalue
within the grid cell occuring most frequently?
Consequently i want only one row for each gid, and the maximum occuring
rastervalue.

SELECT gid, (foo.geomval).val, COUNT((foo.geomval).val) AS ct
FROM (SELECT globshort.rid, priogrid_land.cell, priogrid_land.gid,
ST_Intersection(globshort.rast, priogrid_land.cell) AS geomval FROM
globshort, priogrid_land) AS foo
WHERE gid > 151000 AND gid < 151010
GROUP BY gid, (foo.geomval).val;

gid; val; ct
151001;14;381
151001;150;9
151001;50;7
151001;140;91
151001;40;1
151001;70;2
151001;130;4
151001;200;48
151001;100;3
151001;;0
151001;190;1
151001;20;203
151001;11;111
151001;210;16
151001;30;105

2011/2/23 Paragon Corporation <lr at pcorp.us>

Have you looked at ST_Intersection.  I'm not sure how large your grids are
so might still be a bit too slow.  

http://www.postgis.org/documentation/manual-svn/RT_ST_Intersection.html

Below is a link to our slides from our North Carolina GIS meeting that may
answer some of your questions (shows some Raster examples) as well as the 3D
ones people have asked.

http://www.postgis.us/presentations

Hope that helps,
Regina and Leo

  _____  

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Andreas
Forø Tollefsen
Sent: Tuesday, February 22, 2011 4:28 AM
To: PostGIS Users Discussion
Subject: [postgis-users] ST_Value from Polygon

Hi all, 

I am working with a large raster dataset that i want to aggregate into
vector grids.
The raster dataset is a landcover dataset, and i want to find which of the
raster values are the most dominant within each of the vector grid cells.

I have been looking at the ST_Value function, but this is not usable
together with the cell polygon.

I have written a script that gives me the raster value of the centroid of
each cell, but i want to find which raster class is the largest.
Hence i need to calculate the area of each raster class within each cell and
select the largest class.

Any idea? So far i have only come this far:

DROP TABLE IF EXISTS globshortpoly;
SELECT priogrid_land.cell, ST_Value(rast, ST_Centroid(cell))
INTO globshortpoly
FROM priogrid_land, globshort
WHERE rast && priogrid_land.cell 
LIMIT 1000

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