[GRASS-user] Trouble when I re-project a vector

[Micha Silver]

On 01/19/2011 08:51 PM, Thomas Adams wrote:

>  All:
>
> With GRASS 6.4.0 on RedHat Linux I can successfully import a vector 
> shapefile into a Lat-Long GRASS LOCATION. Then I can display the 
> vector file (stream basins) with a color fill — no problems. What I am 
> trying to do is re-project these basins into a Lambert Conic Conformal 
> (LCC) LOCATION. The vector map seemingly re-projects fine, but when 
> display the vector file (stream basins) with a color fill, not all the 
> basins are filled, apparently because no areas are define for the 
> polygons? How do I correct these basin areas?
>
> Tom
>
Hi Tom:
In order for an area to be a "polygon"  in GRASS it must have both a 
closed boundary and a centroid. When you projected to the LCC location, 
both the boundaries and the centroids *should* have been projected into 
the new vector.  So your new vector should have the same number of both 
areas and centroids as the original.  ( I guess it's theoretically 
possible to have a centroid inside a polygon in one projection, and have 
it fall outside of the polygon when reprojected.)

If you could run v.info on the original vector, and again in the 
projected mapset, you will be able to see if the number of centroids 
matches the number of areas. If not, then you'll have to start the 
digitizer and just add the missing centroids by clicking somewhere 
inside of those areas (basins) that are missing a centroid.

-- 
Micha



-- 
Micha Silver
Arava Development Co. +972-52-3665918
http://www.surfaces.co.il