[GRASS5] r.mapcalc and NULL question
Eric G . Miller
egm2 at jps.net
Mon Jan 15 15:40:21 EST 2001
On Mon, Jan 15, 2001 at 12:41:58PM -0700, Roger S. Miller wrote:
> On Mon, 15 Jan 2001, Markus Neteler wrote:
> > Hi there,
> > maybe a r.mapcalc/NULL FAQ, but I don't get it...
> > Assume you have a streams map (streams contain values,
> > other cells=NULL).
> > Now I want to add some values *everywhere* using r.mapcalc, but
> > it is only added in those cells not containing a NULL value.
> I guess you probably want the NULL-valued cells to be treated like zeros.
> To add maps A and B (where B contains NULLs) to get a map C you can use a
> construction like
> I found a need for this when I used Grass 5 to run the Leicestershire
> tutorial, which was written for Grass 4.x.
> There's another NULL-related behavior that I find rather annoying. I
> guess that the numerical value of NULL is actually -129. I constructed a
> raster map of water-level changes from a ground water model and found a
> band of nulls scattered across the map, wherever the surface passed -129
No, the NULL value depends on the CELL type. Think this is/was a bug in
r.mapcalc specifically (If I recall, it has been corrected). Generally
the embedded NULL values (when the file is read via G_* funcs) are
CELL : MAX_INT
FCELL : NaN (float version)
DCELL : Nan (double version)
However, applications should always use the G_is_null_value() family of
functions. The CELL Null value is problematic if used incorrectly
because you can silently get overflow/underflow errors. NaN used in any
context should always produce NaN (NaN = NaN - NaN) -- at least, that's
how it's supposed to work ;)
Eric G. Miller <egm2 at jps.net>
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