[GRASS-dev] v.category report area

[Markus Metz]

On Tue, Apr 23, 2013 at 3:21 PM, Benjamin Ducke <benducke at 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 at fastmail.fm
>
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