[GRASSLIST:4502] Re: using v.what.rast

[Radim Blazek]

On Wednesday 06 October 2004 05:07, Joel Peter William Pitt wrote:
> Hi All,
>
> I'm trying to use v.what.rast to find out the value of cells in a raster
> map at the coordinates of shapes in a vector file (using grass5.7 cvs)
>
> A couple of problems I'm having are:
> 1. How do I create a new column in a table within a dbf file? Is it
> possible through GRASS? Or should I just use a seperate tool?

echo "alter table xxx add column yyy double precision" | db.execute

> 2. I've tried just overwriting the category( "cat" ) column but nothing
> happens:
>
> 1284 categories loaded from table
> 0 categories loaded from vector
> 0 categories from vector missing in table
> 0 duplicate categories in vector
> 0 records updated
> 0 update errors
>
> Is this because v.what.rast is limited to only points? 

Yes.

> If so, can anyone
> suggest a way of either converting shapes to sets of points, or calculating
> the number of cells of each type under a shape?
>
> Thanks for your time,

Lines and polygons usually cover many different values so v.what.rast
does not make sense. You can overlay 2 rasters.

Radim