[postgis-users] ST_Value from Polygon

Fri Feb 25 00:36:52 PST 2011

Hi Pierre. I did tile it when i processed it.
I used this gdal2wktraster syntax: python gdal2wktraster.py -r
c:\prio_grid\source\globcover\globshort.tif -t globshort -s 4326 -k 720x360
-I -o globshort.sql

The size of my raster is X: 129600 Y: 55800

2011/2/24 Pierre Racine <Pierre.Racine at sbf.ulaval.ca>

> You should just divide the area of your polygon by the area of one of your
> pixel.
>
>
>
> I want to investigate this example a little bit further. What is the size
> of your raster in pixels? I understand that you did not tile it. Did you?
>
>
>
> Pierre
>
>
>
>
>
> *From:* postgis-users-bounces at postgis.refractions.net [mailto:
> postgis-users-bounces at postgis.refractions.net] *On Behalf Of *Andreas Forø
> Tollefsen
> *Sent:* 24 février 2011 09:53
> *To:* Paragon Corporation
> *Cc:* PostGIS Users Discussion
>
> *Subject:* Re: [postgis-users] ST_Value from Polygon
>
>
>
> Great.
>
> Thank you so much. I should have noticed that these were unioned numbers.
>
> Btw. are there any way of getting the count of pixels within a polygon
> without actually aggregating them or union them?
>
>
>
> Andreas
>
> 2011/2/24 Paragon Corporation <lr at pcorp.us>
>
> 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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