[GRASS-user] how to report/measure a vector area?

[Shane Litherland]

Thanks Micha,

I've used v.to.db a bit, just not recently enough to remember that was
my answer! Thanks for your tips/reminder, I should be OK in getting it
to happen for me now :-) If you don't hear from me again on this, I've
succeeded.

regards,
shane.


On Mon, 2012-01-30 at 10:01 +0200, Micha Silver wrote:
> On 30/01/2012 09:52, Shane Litherland wrote: 
> > Hi all,
> > 
> > I'm lost. I have looked around in GRASS for how to report the area (e.g.
> > in hectares) for a vector map that, yes, has area (i.e. centroids and
> > boundaries).
> > 
> > I cannot find a way to do this. A bit of web searching, even gave some
> > suggestions this basic facility may not be available in GRASS?? surely
> > there's something tucked away to do it?
> > 
> > I'm sure I've done something in the past to view vector area data,
> > though it may have been area data that was already in a table. When I
> > use the query tool in map display, it tells me 'nothing'. Seems that is
> > because my vector is not connected to an attribute table. For the
> > purpose of my exercise I did not need cats for the boundaries/centroids
> > so I didn't bother with a table connection either. Maybe this is where
> > I've gone wrong?
> > 
> > I did try vector to raster to try and report on how many raster
> > cells/area... no luck there either.
> > 
> > I also tried the 'measure' tool in the map display, but it only gives
> > distance.
> > 
> > 
> > I could go and learn more about PostGIS for sending the vector data to a
> > postgres geometry table and figure it out that way... I was hoping to
> > find a way to do it in GRASS in a matter of minutes, not via a database
> > and a couple of days/weeks of learning how...
> > 
> > What tool/command should I be using?
> 
> v.to.db is what you need. From the man page:
> 
> 
> Upload polygon areas to corresponding centroid record in the attribute
> table:
> 
> v.to.db map=soils type=centroid option=area col=area_size unit=h
> 
> You'll need to start by adding a database connection and a column for
> area. Again in the man page there's a sample you can follow- just use
> option=area instead of option=sides:
> Upload category numbers of left and right area, to an attribute table
> of boundaries common for the areas:
> # add categories for boundaries of the input vector map, in layer 2:
> v.category soils out=mysoils layer=2 type=boundary option=add
> # add a table with columns named "left" and "right" to layer 2 of the
> input
> # vector map:
> v.db.addtable mysoils layer=2 col="left integer,right integer"
> # upload categories of left and right areas:
> v.to.db mysoils option=sides col=left,right layer=2
> # display the result:
> v.db.select mysoils layer=2
> > regards,
> > Shane.
> > 
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> > 
> 
>