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

[Markus Metz]

On Thu, May 3, 2012 at 12:49 AM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> On Thu, 3 May 2012, Markus Neteler wrote:
>
>> It is in the cats/ subdirectory.
>
>
> Markus,
>
>  Interesting:
>
> # 166 categories
> Watershed basins
>
> 0.00 0.00 0.00 0.00
> dem.basin lines 1-4/4 (END)
>
>
>> They are created if the raster maps has category labels (see
>> r.category).
>
>
>  dem.basin: even numbers 2-166.
>
>
>> GRASS 6.4.3svn (nc_spm_08):~ > v.db.select myzipcodes
>
>
> v.db.select basinv
> cat|value|label
> 2|126|
> 3|126|
> 4|110|
> 5|2|
> 6|2|
> 7|96|
> 8|122|
> 9|20|
> 10|98|
> 11|126|
> 12|60|
> 13|130|
> 14|4|
> 15|58|
> 16|108|
> 17|14|
> 18|120|
> 19|124|
> 20|120|
>
>
>> Maybe too late here, I don't get the last question.
>
>
>  Notice above from basinv: cats 2 and 3 have same value (but r.category
> shows only even number cats for the raster version, dem.basin). Also cats 18
> and 20 have the same value. That's what I mean by my third question: why
> more categories in the vector produced in r.to.vect than in the raster
> source, and why duplicate values for different categories?

If you use r.to.vect without the -v flag, each area becomes a unique
category and the raster value is stored in the column value. If you
use r.to.vect with the -v flag, raster values are used as area
categories.

There are more areas than raster basins because e.g. the basin number 1 in

1 2 2 2
3 1 2 2
3 3 1 2
3 3 3 3

will be represented as 3 separate vector areas.

Markus M