[GRASS-dev] Re: [GRASS-user] vector layer labelling doesn't work....

[Michael Barton]

On Aug 9, 2008, at 5:48 AM, <grass-user-request at lists.osgeo.org> wrote:

> I have no definite opinion on this, but I see Giovanni's point and I
> think the latest patch I posted responds to this: labelling areas when
> areas+labelling is asked for, without showing centroids...
>
> So, let's ask for more opinions: should labelling of areas be linked  
> to
> centroids only, and thus display of centroids obligatory for seeing
> labels for areas, or should labels for areas be displayed even if
> centroids aren't ?
>
> I filed this as an enhancement ticket on Trac:
> http://trac.osgeo.org/grass/ticket/252, so maybe discussion should  
> go on
> there.

Let me suggest something more radical, since we're rethinking this for  
GRASS 7 too.

Do we really NEED an explicit and user accessible vector area  
centroid? What for?

The "centroid" is NOT in the center of an area poygon. It can be  
anywhere inside. It is only a place holder to differentiate an area  
from a boundary. It is a feature of GRASS's internal structure for  
creating vector objects from primitives. It currently also serves as  
the area labeling point, but annoyingly has to be displayed just to  
see a label. But what else does it do from the GIS use point of view?

I suggest the following...

1) Area centroids should always be created by GRASS (not by a user  
clicking a point somewhere in the area), and they should be the  
geographic centroid of the area. In this way, they have topological  
significance and can be used for some vector operations.
2) Area centroids are never displayed by default, but like line  
directions can be displayed and accessed if needed.
3) Area centroids can be associated with labels or not be associated  
with labels. I can see utility of doing either.

Michael