On Jan 29, 2008, at 5:12 PM, Moritz Lennert wrote:
On 28/01/08 16:22, Michael Barton wrote:
On Jan 28, 2008, at 5:50 AM, Moritz Lennert wrote:
On 27/01/08 20:30, Michael Barton wrote:
v.univar only works with points. But since it is calculating
stats on a field in the attributes table, it should work the same
for all vector objects. Can we get rid of the limitation that it
only works with points?
There was some debate [1] about the statistical validity of working
with the other types, as the way it was programmed, the statistics
were calculated with weights which corresponded to line length /
area surface .
I guess we might want to distinguish between a v.univar which works
on the actual vector objects from a v.db.univar which works on any
arbitrary attribute (or combination of attributes). We could write
a C-replacement of the current v.db.univar script on the base of
the code I have for the classification algorithms used in v.class.
AFAICT, v.univar does not calculate anything from vector topology,
only from an attribute column.
[...]
An attribute is the same whether it's linked to a point, line, or
area.
v.univar currently calculates as follows for lines and areas, even
though the results are never printed (main.c):
[lines:]
206 l = Vect_line_length ( Points );
207 sum += l*val;
208 sumsq += l*val*val;
209 sum_abs += l * fabs (val);
210 total_size += l;
[areas:]
270 a = Vect_get_area_area ( &Map, area );
271 sum += a*val;
272 sumsq += a*val*val;
273 sum_abs += a * fabs (val);
274 total_size += a;
285 if ( (otype & GV_LINES) || (otype & GV_AREA) ) {
286 mean = sum / total_size;
287 mean_abs = sum_abs / total_size;
So the mean is actually a weighted mean with the area as weight. I
don't
really no why Radim coded it like this at the time, and I think we
should change this so that it just uses unweighted feature counts,
just
as Roger suggested at the time. Try the attached (untested) patch.
One thing that does potentially matter, though, is whether to use
the features or the attribute columns as a base. If you have
several features with the same cat value, this can make a
difference, as in the former case they will all be counted
individually, whereas in the latter case, they will only be counted
once. If each of the features has an indvididual meaning than the
former case seems more correct, but if not (e.g. each island of the
Philippines counted separately in a table which lists population by
country). Obviously we could just say that it is up to the user to
make sure that the map data is correct, i.e. if we take the above
example, there should only be one centroid linked to data per
country).
The way the routines are written in v.class, they take an arbitrary
array of floats, so it is up to the individual modules to decide
how to create this array.
This is all very interesting. It is a bit worrisome too. I don't want
a mean of an attribute column weighted by area unless I specifically
ask for it. This suggests that people using v.univar may not be
getting what they think they are getting. I think it is an excellent
option, but should not be a silent default.
How to count the features is a bit of an issue, but couldn't this be
left up to the user too--summarize by cat or by individual feature as
an option?
Michael
____________________
C. Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University
Phone: 480-965-6262
Fax: 480-965-7671
www: <www.public.asu.edu/~cmbarton>
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