[Thuban-list] classification of variable ratios, not only single variables

Daniel Calvelo Aros dcalvelo at minag.gob.pe
Sat Mar 6 09:21:37 CET 2004

On Fri, 5 Mar 2004 16:42:34 +0100 (CET), Moritz Lennert wrote
> Hello,
> I'll just keep going until someone stops me ;-)
> How difficult would it be to introduce the possibility of 
> classifying a map according to the ratio of two variables instead of 
> one variable alone. As an example: if I have a map whose data table 
> contains population and surface for each spatial entity, I have to 
> go into the data file (outside of thuban) and calculate the 
> population density and then I can generate a map of this density. It 
> would be nice to have the possibility to indicate two variables, one 
> as nominator and one as denominator and then launch the 
> classification on this ratio.
> Moritz

It wouldn't be hard. The point is to do it right. 

I mean: the fine way would be to introduce a variable calculator into thuban,
and the possibility to add new (transient) variables into a table. Since we
are using SQLite, the operators are there; there is no need for a full
expression parser.

The odd way (to me) would be to use, like ArcView does, a "Normalize by" extra
variable in the classification dialog. I'd rather have the first, although the
second seems more straightforward for interactive exploration.

Just thinking out loud, but what do you all think of having a *postgis*
calculator, including the GEOS-based extensions, so that we could within
thuban have choropleth maps of, say, the length of rivers that are contained
within state boundaries? That's a lot of extra packages needed, and installing
postgis isn't very straightforward right now...

Thoughts? (I'd be glad to start implementing whatever solution is preferred).


PS. BTW, would people at intevation update thuban .deb to 1.0? What about
packaging postgis+geos for debian? And mapserver? And... (we really need a
debian developer with a strong GIS bias...)

-- Daniel Calvelo Aros
-- Dirección General de Información Agraria
-- Ministerio de Agricultura del Perú
-- (51-1)424-9001

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