[GRASSLIST:3992] Re: how to access built-in vector attributes (GRASS 5.7)?

[Markus Neteler]

On Wed, Jul 21, 2004 at 03:59:41PM -0500, William K wrote:
> Or whatever they're called.  Right now I'm looking at lines.  When you 
> d.what.vect, it shows cats, fields and attributes, and length and line 
> height.  I haven't checked polgons yet, but I'm guessing they have 
> perimeter and area instead of length.
> 
> 1. I would like to v.extract to extract lines above a certain length - 
> ie weed out those short blips.

You need to use v.to.db.

1. use db.execute create a new table:
  
 Example:
  echo "CREATE TABLE roadlength (cat integer, linelength double)" | db.execute

  Link this table as second table to your map:

  #check current settings:
  v.db.connect -p roads2
  
  #link new table:
  v.db.connect m=roads2 dr=dbf data='$GISDBASE/$LOCATION_NAME/$MAPSET/dbf' \
               key=cat table=roadlength field=2

2. Verify that the new tables exists now:

  v.info -c roads2
  v.info -c roads2 field=2

3. Generate new categories in the map: 
  v.category roads2 out=roadsnew field=2 option=add
  v.category input=roadsnew option=report

  # should report something like this (for field=2):
  #  type       count        min        max
  #  line         825          1        825
  # -> 825 lines in this map

4. Populate new attribute table with CATs for each vector line:
  (make it corresponding to map)

  v.to.db map=roadsnew field=2 option=cat type=line col1=cat

5. Upload line lengths of each vector line to attribute table:
  
  v.to.db map=roadsnew field=2 option=length type=line col1=linelength units=me
 
6. Check with
   
   d.erase
   d.vect roadsnew
   d.what.vect

7. Extract data (here: shorter than 2000meters):

   v.extract roadsnew out=roadsnew_short  field=2 where="linelength < 2000"
   d.vect roadsnew_short col=red


Note: Initially you may want to use v.build.polylines to build polylines from lines.

> 2. I want to export the lines with line height as an attribute.  Not 
> important now, but labelling lines with line height, using different 
> colors... (not ready to mess with NVIZ yet, but maybe that's the only 
> way?), or extracting certain line heights or ranges of heights.

... see above (now you should be able to do that).

Hope this helps

 Markus