[Thuban-list] classification of variable ratios,
not only single variables
mlennert at club.worldonline.be
Mon Mar 8 11:04:52 CET 2004
Let's keep the discussion going concerning ratios, now that the Debian
issue seems cleared up.
Daniel Calvelo Aros said:
> On Fri, 5 Mar 2004 16:42:34 +0100 (CET), Moritz Lennert wrote
>> 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.
> 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
> and the possibility to add new (transient) variables into a table. Since
> are using SQLite, the operators are there; there is no need for a full
> expression parser.
Could you elaborate on this ? I am not sure I understand correctly. You
mean that instead of having to include all variables into the table, we
should be able to import (transient) variables from other sources ?
> The odd way (to me) would be to use, like ArcView does, a "Normalize by"
> variable in the classification dialog. I'd rather have the first, although
> second seems more straightforward for interactive exploration.
Yes, it is very easy to use. I guess we also have to watch out concerning
the direction of Thuban: it is a viewer not a GIS, or ?
> 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
> within state boundaries? That's a lot of extra packages needed, and
> postgis isn't very straightforward right now...
And it would oblige everyone to use postgis to be able to calculate simple
ratios ? For many usages this seems overkill.
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