[GRASS-user] Relationships of Raster & Vector Attribute Data

[Markus Neteler]

On Wed, May 2, 2012 at 6:21 PM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
>  The basin map output from r.watershed is called 'dem.basin'. When I query
> a basin I see the easting and northing of the point where I clicked, and a
> numeric site_name variable. I've not checked all polygons, but I assume each
> is uniquely numbered.
>
>  First question: where in the mapset's directory structure is this
> site_name variable stored? I've looked in the subdirectories with a
> dem.basin file and do not see them.

It is in the cats/ subdirectory. See the second drawing in
http://grass.osgeo.org/programming6/
for the structure.

>  Converting this raster map to a vector using r.to.vect with an output name
> of 'basinv', creates a postgres table with three attribute columns: cat,
> value, and label. The latter is blank for all rows. The value column,
> however, has the same number repeated for different polygons.
>
>  Second question: how are these value numbers created?

They are created if the raster maps has category labels (see
r.category).

North Carolina example.

GRASS 6.4.3svn (nc_spm_08):~ > r.category zipcodes
27511   CARY
27513   CARY
27518   CARY
27529   GARNER
...
GRASS 6.4.3svn (nc_spm_08):~ > g.region rast=zipcodes -p
projection: 99 (Lambert Conformal Conic)
zone:       0
datum:      nad83
ellipsoid:  a=6378137 es=0.006694380022900787
north:      228500
south:      215000
west:       630000
east:       645000
nsres:      10
ewres:      10
rows:       1350
cols:       1500
cells:      2025000

GRASS 6.4.3svn (nc_spm_08):~ > r.to.vect in=zipcodes out=myzipcodes feature=area
Extracting areas...

...
r.to.vect complete.

GRASS 6.4.3svn (nc_spm_08):~ > v.db.select myzipcodes
cat|value|label
1|27513|CARY
2|27607|RALEIGH
3|27608|RALEIGH
4|27604|RALEIGH
5|27603|RALEIGH
...

Works nicely.

>  Third question: why are there multiple values for different vector
> polygons where there are distinct site_name numeric values in the raster
> map?

Maybe too late here, I don't get the last question.

Markus