[postgis-users] Simple Point Density Surface

[Doug_Newcomb at fws.gov]

>Thanks for the reference Doug. What you suggest does sound like it could
>work, the only thing is that 
>I would need to have this output as a Raster as I am then looking to use 
it
>with two other rasters 
>to do a weighted combination of them e.g. [rast1 * 0.2] + [rast2 * 0.5] +
?[rast3 * 0.3].

>Do you know how if I could easily rasterise the output of what you've
>suggested?

James,
Unfortunately, I have not waded into rasters inside postgis yet.  I'm 
speculating that you could generate a raster from the point values you 
generated from the suggested calculation.  ( Using St_AsRaster, 
http://postgis.refractions.net/docs/RT_ST_AsRaster.html ?) 

For raster algebra, I usually use GRASS r.mapcalculator - 
http://grass.fbk.eu/gdp/html_grass64/r.mapcalculator.html.  If you can 
dump your images out to files, you could import them to grass ( or use 
r.external to link to them) and perform the calculation you described.  If 
you want to stay in postgis, just do the calculation in two steps.     

>Only reason is I'm working on it for a dissertation project and I've left
>myself little time left to consider this.
Sounds like my grad school experience :-)

Doug


Doug Newcomb 
USFWS
Raleigh, NC
919-856-4520 ext. 14 doug_newcomb at fws.gov
---------------------------------------------------------------------------------------------------------
The opinions I express are my own and are not representative of the 
official policy of the U.S.Fish and Wildlife Service or Dept. of the 
Interior.   Life is too short for undocumented, proprietary data formats.



JamesH <james.holmes at newcastle.ac.uk> 
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04/25/2012 02:49 PM
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Re: [postgis-users] Simple Point Density Surface






"Why not convert the raster cell centers to points , do a query on the 
point layer for a square centered on the raster derived point  whose 
dimensions are given by the number of tiles in your neighborhood.  return 
the number of points divided by the area of the square.  ( neighborhoods 
are usually odd numbers of cells, 3x3, 5x5, etc so you have an equal 
number around the center cell)"

Thanks for the reference Doug. What you suggest does sound like it could
work, the only thing is that 
I would need to have this output as a Raster as I am then looking to use 
it
with two other rasters 
to do a weighted combination of them e.g. [rast1 * 0.2] + [rast2 * 0.5] +
[rast3 * 0.3].

Do you know how if I could easily rasterise the output of what you've
suggested?

Only reason is I'm working on it for a dissertation project and I've left
myself little time left to consider this.

Kind Regards,
James 


-----
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