[GRASSLIST:8464] Re: extracting the value of a raster cell

[Jonathan Greenberg]

    Check out starspan: starspan.casil.ucdavis.edu -- some day we might work
on folding this into GRASS, but until then its an incredibly efficient way
to extract raster values from vector layers and do some stats on them...

--j


On 9/30/05 9:56 AM, "Shaun Walbridge" <walbridge at nceas.ucsb.edu> wrote:

> Jason,
> 
> GRASS doesn't have much native support for geostatistical calculations
> such as spatial mean, but it should be a fairly trivial task if you use
> the R/GRASS interface.  A few of my co-workers use R in this fashion,
> with generally great success. A primer on the topic is available at:
> http://grass.itc.it/statsgrass/grass_geostats.html.
> 
> - Shaun
> 
> Jason Horn wrote:
> 
>> Thank you Hamish, for the response.
>> 
>> As usual, I don't think I was specific enough. What I'm looking for
>> is a way to extract the value of a particular raster cell.  r.what
>> will do the trick, but only if you know the exact coordinates.  I
>> want to loop through in a script and extract the value of each cell
>> in a raster coverage.  For example, if a raster layer was a 10x10
>> grid, I want the value of cell (1,1), then cell(2,1), then cell
>> (3,1), etc..  If I were to use r.what, calculating the location of
>> the next cell using resolution and the coordinates of the first cell,
>> I might accidentally skip a cell due to rounding errors.  r.mapcalc
>> allows you to access the value of a cell directly, but the result is
>> always another raster map - you can't output the cell value directly.
>> 
>> The reason for all of this is that I am trying to write my own script
>> for calculating the spatial mean of a given raster map.  I am
>> surprised that there is no facility for this in GRASS already - it
>> seems like such a basic spatial statistic.  My solution, so far, is
>> to output the map as an ascii file, then load the ascii file into an
>> array within a script.  It works, but seems like a ridiculous
>> workaround for something so simple.
>> 
>> If anyone can think of another way, please post it.  All suggestions
>> welcome.
>> 
>> I guess I should probably write my own module in C, but I've never
>> had much luck getting GRASS to compile successfully in the first
>> place, so I'll stick with the script route for now.
>> 
>> - Jason
>> 
>> 
>> 
>> On Sep 28, 2005, at 7:57 PM, Hamish wrote:
>> 
>> 
>>>> Does anyone know how to extract the value of a cell at given
>>>> coordinates from a raster layer?
>>>> 
>>>> 
>>> 
>>> r.what
>>> 
>>> 
>>> 
>>>> I am writing a script that will calculate the spatial mean for a  given
>>>> raster layer. Therefore, I need to be able to access the values of
>>>> raster cells.
>>>> 
>>>> 
>>> 
>>> r.univar?
>>> 
>>> If you want the stats within a buffer around a point, I would  suggest a
>>> vector point map -> v.buffer -> v.to.rast -> MASK map -> r.univar in a
>>> loop, or probably simpler with Markus's new v.rast.stats module in
>>> 6.1-cvs.
>>> 
>>> 
>>> 
>>> Hamish
>>> 
>>> 
>>> 
>>> 
>> 
>> 
> 


-- 
Jonathan A. Greenberg, PhD
NRC Research Associate
NASA Ames Research Center
MS 242-4
Moffett Field, CA 94035-1000
650-604-5896
AIM: jgrn307
MSN: jgrn307 at hotmail.com