On 30/01/08 02:43, Michael Barton wrote:
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.
Well, since the results are not printed, the problem doesn't really
exist. The patch I sent doesn't weight at all, just counts features.
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?
That's why I think we should have a library function which calculates
stats (i.e. extend what it is the v.class code), and let the modules
deal with such issues.
Moritz
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