Translating file formats

[Rich Shepard]

On Thu, 18 May 2000, David D Gray wrote:

> > >   The other hang up is my understanding of the "arc-node" format in
> > > practical terms. A MI region (polygon) is a list of nodes with the first and
> > > last nodes having identical values. I've no idea how to separate this one
> > > line into meaningful segments (arcs) for input into GRASS. I'm certainly
> > > open to ideas, thoughts or solutions, for I've never had to deal with this
> > > before.
> > 
> > A line segment is a remapping of a start node,
> > a series of vertices and an end node. Polygons are mappings of line
> > segments (arcs) -- minimum of 3.  So a particular node can be shared by
> > multiple objects, and the same for a particular arc (or edge).  Note, a
> > node has a particular definition as having at least 3 connecting line
> > segments.
> 
> So I think we have the basis for dealing with imports of `geometric'
> format coverages. Also location of area/label points and attribution
> (also postgres support for multiple attributes thanks to Alex
> Shevlakov).

  I understand the concept of starting at a node, walking the intermediate
points until another node is reached, and so on. But, I don't understand how
to do this in practice.

  I just exported a MapInfo polygon file: counties in the state of Nevada.
There are only 16 counties so the file is relatively small. The attributes
for each polygon is the county name, county number and state number. The
attributes are held in the *.mid fil, the graphics in the *.mif file. There
is a one-to-one correspondence by order. That is, the fist graphic unit is
associated with the first attribute, and so on.

  Rather than attach both files and send them to everyone on the mail list,
I'd rather work with those more knowledgable and experienced than I off the
list. I can explain each element in the .mid and .mif files. What I really
want to learn is how to parse these objects into the arc-node format of
GRASS and how to then build the topology.

  I'd also appreciate pointers to references where I can learn more about
the storage of spatial topology. I understand quad trees (although it's been
almost a decade since I did any reading on them).

Thanks,

Rich

Dr. Richard B. Shepard, President

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