[GRASS5] vector import formats

David D Gray ddgray at armadce.demon.co.uk
Tue May 9 15:46:25 EDT 2000

Hi everyone

This is my first posting to this list, though I have 
been following the discussions for a month or so. I
have some questions about vector import modules:

Many of our import modules produce maps where the linework
is not in the `proper' format. GRASS seems to be designed
to work with lines in the `arc-node' format, which is
also like the 2-dimensional manifold definition from the
SDTS spec. I hope we will retain this, as this format is
optimally designed for line editing and topological
relationships between vetcor GIS objects. But where the
lines are imported as closed loops for example, or 
other non-native formats, GRASS cannot process the
topology correctly as the data is ambiguous or
meaningless ( what is the left polygon of an arc
that is a closed loop?). 

I suggest we class imports that are 
incorrect this way as a bug and modify modules that produce
these maps to provide a correct arc-node representation.
I am currently working on this with the shapefile import
as this is the most urgent for various reasons. But v.in.arc and
v.in.dxf also do not always import maps correctly,  and possibly
there are others. 

Or putting it another way: does anyone know of any real
advantage to having vector maps imported with closed rings, so
that we would want to keep our import modules able to
create these formats?

David Gray

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