[GRASS-dev] C replacement for d.vect.thematic

[Florian Kindl]

Jerry:
>> Personally I prefer a constant color gradient, not stepped/binned
>> legend like Arc produces. I consider that to be a relic from the days
>> of 16 or 256 max color pallets. Of course I work with mostly FP data
>> and not categorical, so I'm biased. I guess the legend should follow
>> the nature of the data, be it categorical, a continuous linear range, a
>> continuous logarithmic range, a +/- differences map centered on 0, etc.
>> Anytime you introduce an unneeded histogramming step you are getting
>> dangerously near to writing a new chapter for "How to lie with
>> statistics" (great book).

Moritz:
> Interesting point. Will have to think about it. In human geography 
> categorising data to display in discrete classes is the usual way of doing 
> things. But I don't know whether there are any other reasons for it than 
> technical relics and a desire for simplification. In any case, I think that 
> discrete ranges should still remain a possibility.
>

Hello,
I'd like to give my opinion on this from a cartographic point of view:
Forming classes by assigning the same color to a range of values does
not come from any technical limitations. It is an established and
necessary technique in cartography.
The reason is to make a map a) readable and b) not to pretend accuracy.

ad a):
Even though the computer can distinguish millions of colours and screen
or print devices technically may be able to reporoduce several hundreds
of thousands, this does not allow for the use of so many shades for a
map. The reason for this is that the human eye can only distiguish some
tens of different shades, and about ten or so of the same hue.

ad b):
The use of a "pseudo-continuous" (using so many shades that we can't see
the difference, you get the crucial point here?) color ramp tries to
mimic a high level of accuracy in the data presented. However, that
accuracy does not reach the reader of the map as we cannot reliably
connect one position on the color ramp to a given pixels color on the
map. 
Additionally, the value resolution of the color ramp could exceed
the precision of the values. 
For example, a DEM has values from 0 to 1000 meters. You colour it with a 
ramp of 100000 shades, therefore claiming an accuracy of height of 1 cm in 
your map. Unlass you have such precise data, that is just nonsense. Even
if one was able to distinguish one value from the other.


To address your concern of "lying with statistics":
It lies in the responsibility of the cartographer to make a map which
conveys its message in the best way. Of course it is possible to make
very biased maps by categorizing values. How the map-maker decides this 
is a matter of his own goals and ethics, which I cannot judge here, and a
question of following cartographical rules, which you find condensed in
the any textbook on cartography.


To conclude:
If a GIS wants to produce output that possesses cartographic quality,
the possibilibity to categorize cannot be left out.


Sincerely,
Florian Kindl




-- 
Mag. Florian Kindl
Geographie Innsbruck

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