[GRASSLIST:18] Re: r.mapcalc limited number of input maps?

[Glynn Clements]

Otto Dassau wrote:

> > > I would like to add raster map values with r.mapcalc. Does there exist a
> > > maximum number of maps to calculate?
> >
> > AFAIK, there is only GRASS' limit on the maximum number of open maps
> > (256). r.mapcalc doesn't have any fixed limits, but the GRASS
> > libraries on which it depends might.
> >
> > > I tried with 8 maps and it works well, but there is a problem when I want
> > > to add e.f more than 30 maps.
> > >
> > > r.mapcalc "newmap = map1+map2+map3+map4+..."
> >
> > What's the problem, exactly?
> 
> I would like to add a number of maps and it doesn't work with r.mapcalc for a 
> large number of input maps but with r.series it is ok. The maps are stored in 
> a variable $INPUT and it looks like this:
> 
> echo "$INPUT"
> map1+map2+map3+map4+map5...
> 
> the r.mapcalc command is:
> r.mapcalc "output.$TIMESTAMP = $INPUT"
> 
> r.mapcalc calculates without errors but the resulting map is empty when the 
> number of input maps is high. I don't know the exact number of maps, when it 
> works and when not. I only tested with 8 and with 30.  8 is fine 30 not.

I can't reproduce this using simple generated data.

Do the input maps contain nulls? In r.mapcalc, the result of an
addition (or any other arithmetic operator) is null if either input is
null. Hence, a cell in the output map will be null if the
corresponding cell in any of the input maps is null. As the number of
input maps increases, the proportion of null cells will increase.

Note that the r.mapcalc command is equivalent to using r.series with
the -n switch (propagate nulls). Without that switch, r.series only
uses non-null cells in its calculations.

-- 
Glynn Clements <glynn at gclements.plus.com>