[GRASS5] how to NULL in site lists?

Roger Bivand rsb at reclus.nhh.no
Wed Jan 24 02:47:33 EST 2001

On Tue, 23 Jan 2001, Eric G . Miller wrote:

> On Tue, Jan 23, 2001 at 09:59:47PM +0000, Markus Neteler wrote:
> > Hi all,
> > 
> > as working with sites at time, I face the problem that I
> > don't knwo how to code null values in a site_list.
> > (Eric, we already talk once about this weeks ago).
> > 
> > As we cannot simply invent a special character for
> > this (maybe to complex to change the entire sites lib),
> > what about using for GRASS 5.0 the value -0.0?
> > 
> > Just an idea. Or is there another way? 
> > Using -9999 like ESRI does is of course unwanted and
> > currently leads to nonsense results (as it is treated
> > as true value).
> > 
> > As -0.0 is currently also treated as true value, I
> > would like to know if it could become a workaround unless
> > it is improved in 5.1?
> We've discussed this before.  I don't know that modules will treat -0.0
> appropriately (most likely they'll just treat it like zero).  IMHO, the
> whole conception of the sites format sucks from both a user's and a
> programmer's perspective.  When we get proper tabular data support built
> into GRASS, this problem should be easy to handle.  But that's going to
> be a while, and I for one have yet to put my "money" where my mouth is
> ;)  Hopefully, in 5.1 we can solve this.  I don't think using -0.0 will
> do much at this point, since most sites modules have some fundamental
> assumptions about the data they are reading (which includes it not being
> NULL and not being NaN).  Also, this doesn't address NULL string values.
> Sorry if I'm kind of a spoiler on the quick fix, but I think fundamental
> changes are required.
I agree with Eric: the quick fix probably won't be "quick" for all the
programs using sites files. The quickest fix does though - multiple sites
files with single attributes and no NULL or NA values, i.e. subsetting the
sites. A more general solution will, I think, be reached with sites/vector
attribute tables, say by 5.1.


Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
e-mail: Roger.Bivand at nhh.no
and: Department of Geography and Regional Development, University of
Gdansk, al. Mar. J. Pilsudskiego 46, PL-81 378 Gdynia, Poland.

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