[GRASS5] line types

[Eric G. Miller]

On Mon, Apr 08, 2002 at 12:25:31PM -0700, rgrmill at rt66.com wrote:
> Radim wrote:
> 
> 
> > Lines can enclose areas? What do you mean? That grass type LINE can
> > be area edge? No, it can't, areas are build only form AREA type.
> 
> Sorry then.  My misunderstanding.
> 
> > How do you want to remove distinction?. Replace all  "if (Line->type 
> == 
> > LINE)" by  "if (Line->type == LINE ||  Line->type == AREA )
> > or merge  LINE and AREA types in the vector library?
> > 
> > I think that LINE/AREA distinction should be keept and boundary
> > (area edge) option should be add to modules 'type' option.
> 
> In both of the cases that caused me problems the code tested for the 
> line type and skipped the operation (either extracting or labelling) if 
> the line was typed as an area edge.  v.llabel doesn't offer a separate 
> option to label area edges.  v.extract doesn't extract area edges based 
> on the category of the line, only on the category of the area the lines 
> define.
> 
> In both cases it is sufficient to change the code to perform the 
> operation regardless of whether the line is typed as an area edge or as 
> a line.

I'm not sure if this is acceptable behavior for most people.  How do you
distinguish the case where people only want the labelled LINE types as
line, and not area boundaries?  The handling in GRASS hasn't been too
good for mixing types in one map (even though it is allowed).

Perhaps a flag to toggle behavior?  Does v.support accept this?

> As for the general usefulness of the distinction...  I don't know how 
> you can tell whether a line is an area edge or not when you import a 
> vector file.  As a result, I type everything as an area edge.  I don't 
> think it is reasonable to expect a user to manually identify which lines 
> out of tens of thousands of possibilities might make an edge of one of 
> thousands of different areas that may need to be defined.

Your right that it shouldn't matter to users.  All that should matter
is, does an edge participate as an area boundary?  Does the edge
participate in a line feature?  Unfortunately, it doesn't change any
of the architechtural shortcomings of < 5.1 vectors.

-- 
Eric G. Miller <egm2 at jps.net>