[GRASS5] Re: d.vect.thematic wish: handle null values

Moritz Lennert mlennert at club.worldonline.be
Sun Aug 28 05:53:34 EDT 2005

On Mon, August 15, 2005 20:50, Daniel Calvelo said:
> Hi all,
> Sorry for jumping in so late, I've been traveling.
> AFAIR the problem is actually within db.select's output. Moritz, could you
> apply this patch to v.univar.sh and send the result (in /tmp/db.select.out)
> back to me?

done off-list.

> When I was dealing with this problem I only used the DBF driver,
> which gave zeros for nulls under unspecified cirsumstances.

Thinking about this a bit more this seems more like a bug to me. Making nulls
into zeroes gives very different results than if nulls are excluded from the

At the user should be warned about this.
As I mentioned earlier, we have to think a about whether it is better to
silently exclude nulls, or whether we should explicitely give the opportunity
to exclude a certain value (or range or values) from the classification. This
would then allow those with nulls in their files to transform them to any
value (e.g. -99999) and exclude it from the calculations.

> I can fix the script to filter out nulls, but I'm pretty sure the problem will
> remain under certain configurations.

Maybe above solution is more generic. But in any case nulls should be handled
more gracefully...or at least a warning included in the man page...


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