[GRASS-user] GRASS63cvs v.rast.stats reports zero for many CATs

[Dylan Beaudette]

On Friday 06 April 2007 13:36, Brandon M. Gabler wrote:
> Fellow GRASSers -
>
> I am using GRASS 6.3 cvs, the Kynge's build, and I have an issue
> using v.rast.stats on a raster of slope. Many (n>100) of my
> archaeological sites, which are all polygons, produced 9 columns of
> zeros for the slope stat calculations. I repeated the exercise, using
> different column names, and it repeated the results.
>
> The oddest part is that I successfully calculated elevation stats
> from a DEM raster using the same v.rast.stats command. The slope
> raster was produced directly from the DEM, so resolution, etc. are
> exactly the same.

Hi,

r.slope.aspect will create maps with null-cells in some cases- this is a 
feature not a bug, see the manual page. Perhaps the input DEM did not have 
any null cells at these areas, and thus had good results from the trouble 
areas...

> I investigated some of the polys that were missing slope information,
> and indeed, the raster is completely intact in the area of these
> polygons, with values other than zero, and the polygon itself is clean.

were there any null cells?

> In attempting to do the same calculations with my aspect raster,
> there were also many sites where the aspect calculation resulted in 0
> (for both of these, even the 'n' field, number of cells, is 0). These
> sites were not always the same as the ones that had slope
> calculations of zeros!
>
> I am running the stat calculations with slope again, after cleaning
> the vector file, but I still am not optimistic that this will help
> since my v.rast.stats calculations on elevation were successful where
> the slope ones were not.
>
> Also, it takes about 7 hours to calculate the stats for my 893
> polygons (16 meter grid resolution probably has something to do with
> it, but wanted to make sure this isn't a fixable thing).

While there are some solutions to the general problem of 'zonal statistics' in 
GRASS, I tend to use the 'starspan' application for these operations. It is 
VERY fast, and has been used in numerous papers. It can directly read GRASS 
raster and vector data, which is a definite plus. 

google starspan for some ideas. also- matt perry's blog has some examples if I 
am not mistaken.

cheers,

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341