[GRASS-dev] more help with v.category...

[Isaac Ullah]

Hello list. I am continuing to struggle with update/manipulation of vector
categories between the dbf and vector file. Perhaps if I state what I am
trying to do, someone can help me out. I am using latest wingrass 6.4 binary
on XP.

I have some vector points file created by v.mkgrid where the points are
regularly spaced at various grid intervals and orientations. I have used
v.buffer to make a new vector file from these points where there are small
circular buffers around each point. I have a few base vector points maps
that I genertated using several tools (including v.random, v.perturb, and
others) where there are clusters of points at varying densities across the
region. These base maps are to simulate the spread of small pieces of refuse
on an archaeological site, and the grids of circular buffers represent
different sampling strategies. The idea is to be able to count the number of
objects within each sample circle and use that to interpolate a density
surface (first change the random points base map to raster and then use
v.rast.stats with the sample circles map so that count will be in column
"n"). The resulting density surfaces will be compared with the real point
density map to see which sampling strategy works best.

My main problem at the moment is that I need to have a row for each sampling
point/circle in the dbf of the sampling maps. The points in the points maps
made by v.mkgrid each have their own cat number. After running v.buffer to
make circular areas around each point, all circular areas have cat=1. Each
circle has it's own unique feature ID, but all have category = 1. I know I
should to be able to make each vector object have it's own unique category
number. Then I could run v.to.db and update the cat column in the dbf with
these unique cats, and I would be good to go forward from this step. But I
can't get the circles to have unique cats!!!! I've run v.category in every
way imaginable, but I cannot get it to do what I want! It continues to
produce new maps where all the vector points or centroids all share the SAME
cat number! I'll post the output of v.category below. If v.category doesn't
work to do this then how can I get sequential cat numbers? This seems like
such a simple simple thing. Shouldn't be possible to somehow just read the
feature ID's into the category for each object? Please let me know if I'm
doing something wrong, or let me know if GRASS just can't do this.

Okay, here is the output of v.category. First report existing cat numbers:

v.category input=tril_lattice_1m_eq_5cm_RAD_CIRCS at 1m_trilatticeoption=report
type=centroid
Layer/table: 1/tril_lattice_1m_eq_5cm_RAD_CIRCS
type       count        min        max
point          0          0          0
line           0          0          0
boundary       0          0          0
centroid     159          1          1
area           0          0          0
all          159          1          1

See: all have category = 1. Okay now try to overwrite cats following the
logic from the man page ("If the *type* parameter is set to *centroid* and
the *option* parameter set to *add*, new categories will be added to
existing centroids."). So I do this:


v.category --overwrite
input=tril_lattice_1m_eq_5cm_RAD_CIRCS at 1m_trilatticeoutput=tril_lattice_1m_eq_5cm_RAD_CIRCS_cats_added at 1m_trilatticetype=centroid
Vector map <tril_lattice_1m_eq_5cm_RAD_CIRCS_cats_added> already exists and
will be overwritten
Processing features...
Copying attribute table(s)...
Building topology for vector map
<tril_lattice_1m_eq_5cm_RAD_CIRCS_cats_added>...
Registering primitives...
318 primitives registered
3975 vertices registered
Building areas...
159 areas built
159 isles built
Attaching islands...
Attaching centroids...
Number of nodes: 318
Number of primitives: 318
Number of points: 0
Number of lines: 0
Number of boundaries: 159
Number of centroids: 159
Number of areas: 159
Number of isles: 159
v.category complete. *0 features modified.*

It does nothing. I emboldened where it says "0 features modified". To be
sure, I run report on the new vector map:

v.category input=tril_lattice_1m_eq_5cm_RAD_CIRCS_cats_added at 1m_trilatticeoption=report
type=centroid
Layer/table: 1/tril_lattice_1m_eq_5cm_RAD_CIRCS_cats_added
type       count        min        max
point          0          0          0
line           0          0          0
boundary       0          0          0
centroid     159          1          1
area           0          0          0
all          159          1          1

See. It keeps all category = 1!

So, how can I make all 159 individual areas have unique category
numbers?!?!?



PS. As an aside, now that I have for the first time really started to work
with the vector tools in GRASS, I am noticing that they are very very
confusing to use. The documentation is a bit sparse, and the interfaces
don't make much intuitive sense. And it is very difficult to do simple
things like make an attribute table with rows for each vector object. These
kinds of basic operations take multiple steps and sometimes I just can't
make them work...

Cheers,

Isaac

-- 
Isaac I Ullah, M.A.

Archaeology PhD Candidate,
ASU School of Evolution and Social Change

Research Assistant,
Mediterranean Landscape Dynamics Project
***************************************************
isaac.ullah at asu.edu
ullah at archaeologist.com

http://www.public.asu.edu/~iullah
***************************************************
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