On Tue, Apr 23, 2013 at 3:21 PM, Benjamin Ducke <benducke@fastmail.fm> wrote:
Hi,
On 04/12/2013 03:05 PM, Martin Landa wrote:
Hi all,
`v.category` (G7) reports categories also for areas.
v.category rb opt=report
Layer: 1
type count min max
point 0 0 0
line 0 0 0
boundary 0 0 0
centroid 1 1 1
area 1 1 1
face 0 0 0
kernel 0 0 0
all 1 1 1
Categories are assigned only to the centroids. Areas (topological
composition of boundaries and centroid) shouldn't be reported by
`v.category`, I would say (?)
Martin
<it seems to me that there hasn't been any reply
to this by someone more qualified then myself(?)>
My understanding is the same as Martin's:
in the case of areas, categories are attached to
centroids, so the above output is a bit confusing.
From a user perspective (especially users used to simple feature
points, lines, polygons), GRASS vector modules should IMHO by default
work with points, lines and areas. Boundaries and centroids are
special cases. Thus I would suggest to change the default type answer
for v.category (and other vector modules where possible) to
point,line,area and report categories only for the requested types.
The output would then become
v.category rb opt=report
Layer: 1
type count min max
point 0 0 0
line 0 0 0
area 1 1 1
Boundaries and centroids can still be requested with the type option.
Markus M
However, since "areas" do not physically exist in
a GRASS map (only the centroids and boundaries do),
there is a problem here: what if some centroids are
without a category, but still part of an area?
What if there are centroids with categories that are
not placed correctly within a boundary? This
could happen in incomplete or incorrect maps.
And in these cases the difference between the
stats for "centroid" and "areas" can be important
information.
Perhaps instead of always outputting the "area" line,
the module should only output an extra line (or even
a warning) if there is a mismatch between the number
of categorized areas and centroids.
Best,
Ben
--
Martin Landa <landa.martin gmail.com> * http://geo.fsv.cvut.cz/~landa
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--
Dr. Benjamin Ducke, M.A.
{*} Geospatial Consultant
{*} GIS Developer
benducke@fastmail.fm
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