[GRASS-user] Problem running v.rast.stats - caused by calculating coefficient of variance when mean=0?

[Hamish]

Tim wrote:
> I am trying to use v.rast.stats in GRASS 6.4, but cannot get it to
> work with the vector file I am using.  Vector data I am using is
> administrative boundaries (provinces of Vietnam) with the raster
> being forest cover data at 500m x 500m pixel size.  
> 
> Interestingly, it works fine when I use certain subsets of
> my vector file (various sets of provinces selected at random), just not
> when I use the whole file (i.e. all provinces).  When I use the
> complete provinces file, it finishes with errors (copied below).
>  When I check the attribute table, it has created the 9 columns it
> is supposed to, but they are all blank.  
....

> Updating the database ...
> ERROR: This should not happen: wrong return type in
> parsing.
> dbmi: Protocol error
> ERROR: Error while executing: 'UPDATE NorthEast_NoIslands
>        SET TrChng_cf_=inf WHERE cat=3'

....

> Because of the error message ("TrChng_cf_=inf WHERE cat=3",
> I think the issue may have to do with calculating the coefficient
> of variation when the mean of an area is equal to zero.
> The precision of this data is not particularly high, and pixels
> with value zero are common, so with many small islands, the likelihood
> of a couple of them having a mean of zero is quite high.  
> 
> If v.rast.stats (which performs its calculations by calling r.univar,
> I believe)

yes

> is unable to calculate the coefficient of variation in one category
> because of a mean of zero, does that cause the rest of the run to be
> unsuccessful?  Is that what is happening here? 

#test
r.mapcalc zero=0
r.univar zero

sets cf_var to nan for me.

If I try to update a DBF table with a nan or inf into to a double column:
v.db.addcol local_bugs col='cfv double'
echo "UPDATE local_bugs SET cfv=nan" | db.execute
echo "UPDATE local_bugs SET cfv=inf" | db.execute

both those work for me as far as I can tell (v.db.select shows those
columns as empty)


> If so, is there any way around this?  I am not even interested in the
> coefficient of variation, as I am only looking for very basic stats
> (n, mean, max, min).  Is there any way to tell it to skip the CV, or
> is there another work-around anyone can think of?

to test your theory: find and edit the v.rast.stats script,
find this line:
  BASECOLS="n min max range mean stddev variance cf_var sum"
and remove cf_var from it.

>  Or is this some other problem entirely?  

could be... but the "inf" is definitely an obvious suspect.


also you might try changing to the SQLite driver instead of the less
flexible DBF one.  (see db.connect help page)


Hamish