[GRASSLIST:2005] Re: overlay problems in 5.7

[Radim Blazek]

On Tuesday 09 December 2003 21:09, Christopher Fonnesbeck wrote:
> I am trying to clip a vector map to a region using v.overlay, but am
> running into a problem. I have created the region using v.in.region,
> and run this in v.overlay with the vector file I am clipping (see
> attached screen grab). The function seems to run fine, and creates a
> vector file, but when I try to draw this new vector map, it comes up
> blank with no error messages. What am I doing wrong? I did the
> following:
>
> g.region vect=ugr_streams
> v.in.region type=line output=ugr_region       <-- I have tried this
> with type=area as well
> v.overlay ainput=roads binput=ugr_region output=roads_ugr operator=and


'atype' for v.overlay may be either 'area' or 'line' and default is area. 
Input category for area in the input vectors must be defined 
("... Input feature is considered to be true, if category of given field is defined ...")
I see in the picture that there are also areas in 'roads', but maybe without category.
'btype' is always 'area', so v.in.region should be used with 'type=area'. 

I guess, that you have lines and not labeled areas in your 'roads'
and because default 'atype=area' nothing is written to output.
Try to run v.overlay with 'atype=line' or assign a category
to areas in 'roads' (v.category type=area).

What is the output of v.info map=roads ?

Radim