[GRASS5] r.mapcalc and NULL question

[Markus Neteler]

On Mon, Jan 15, 2001 at 02:44:47PM -0800, Eric G . Miller wrote:
> On Mon, Jan 15, 2001 at 03:07:48PM -0700, Roger S. Miller wrote:
> > 
> > On Mon, 15 Jan 2001, Markus Neteler wrote:
> > 
> > 
> > > Generally I consider this as a bug because you can't always know if
> > > a map contains NULLs. If only a few pixels would be nulls you may
> > > use option 2 and don't realize that the result is not want you want.
> > > I feel this behaviour is somewhat dangerous. The isnull() is useful,
> > > but if you have to apply it on default in if-clauses, it's a bit annoying.
> > >
> > > Other comments?
> > 
> > I first felt that it was a bug, but the best alternative I could required
> > a default value to be used in the place of null.  Zero is the most obvious
> > choice for a default value, but I could think of plenty of cases where
> > that wasn't a good idea.  Requiring the user to provide a value for null
> > also seemed like a bad idea.  So I decided it must be a feature.
> > 
> > For what it's worth, Golden Software's Surfer package (and Topo before
> > that) has for years used a value of 1.70141e+038 as a null value -- they
> > use it for "blanking".  Their grid math calculations treat blanks pretty
> > much the same way that r.mapcalc treats null; any operation on a blank
> > cell returns a blank cell.
> 
> I tend to agree with Roger here.  There doesn't seem to be an obvious
> way to handle the scenario, so trying to outsmart the user may create
> more problems than solutions.  Maybe we need to get the word out better
> than any math performed with NULL values should always result in a NULL
> value.  This is the very definition of NULL as an "undefined/unknown value".

Hi Eric, hi all,

I have added such a comment to html/html/r.mapcalc.html
Will be uploaded to the web site this night from CVS by
the web server's cronjob.

BTW: My flooding script is working by using the isnull() function.

Thanks for this discussion!

 Markus

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