[GRASSLIST:8475] Re: extracting the value of a raster cell

[Hamish]

> > Can you explain what is meant by the "spatial mean" calculation?
>
> A spatial mean is simply the mean value of series of numbers that  
> also has a location in coordinate space.  For example, imagine the  
> population of each state in the continental US.  You can easily  
> calculate the mean population size, right?  Ok, now WHERE is the mean 
> population on the map of the US?  It's actually simple to calculate.  
> The spatial mean is really just a weighted average using the  
> coordinates of the center of each state.  So, to calculate the  
> spatial mean for a raster map, you need the value and coordinate pair 
> for each cell.  Hence, my original request.


Thanks for explaining that. So the "answer" is a spatial coordinate at
the "center of mass"; not a number in units of the attribute, correct?

No, I don't think anything exists in grass to do this, but probably the
most efficient way to get the values out using existing tools is:
(Short of writing a custom C module that is)

  r.stats -g input=raster_map

alternatively, 

  r.to.vect(points) + v.out.ascii.db.

v.out.ascii.db is available at the GRASS Wiki add-ons page, I've not
tested it for a dataset as large as r.to.vect(points) will generate.
I'd be interested to know if it works.


But who knows, maybe there's a smart way to use r.cost + r.mapcalc in an
inverse way to find a maxima who's position could be queried.



Hamish