[GRASS5] v.in,dxf

[Roger S. Miller]

On Mon, 21 May 2001, David D Gray wrote:

> Michel Wurtz wrote:
> >
> > I'll probably look stupid, but what is the second phase of the import ?
> > The process of importing vector is mainly a stream (with some exceptions
> > when reading the imput file).  If not, you will need a temporary storage
> > format (on disk, because storing everything in core memory will allways
> > limit the size of the imput file)
> >
>
> It's only a direct import if the original format supports component
> arcs. Many formats record the data as whole polygons, so must be 'split'
> to arcs before writing to dig files, so some kind of intermediate is
> necessary.
>
> The first stage is substantially diffferent for different formats. Also
> there is often pre-processing or different fields to be read. A Mapinfo
> import has to record the Pen and Brush attributes (or will if we are
> going down the road of incorporating these). More significantly, you
> have to `deconstruct' the topology of a set of components in a shapefile
> as a shape can be compound. So stage 1 reads the original format,
> pre-processes the data and deals with spatial and data attributes. Stage
> two imports the linework as arcs regardless of the original format, from
> a common intermediate.

I find this a little confusing for the case of a dxf import.  DXF files
contain no topology, but they do contain a large number of different kinds
of drawn objects, none of which are polygons and none of which interpret
to GRASS polylines without some computation.

I nearly asked the same question that Michael asked, but held off.  Before
reading your answer I had envisioned a five step process.

1)  Read the vector data from it's native format
2)  Recalculate the lines specified in the native format to GRASS
polylines, and store the raw data.
3)  Check each object against objects already imported and remove
non-unique object
4)  Build GRASS database support files
5)  Write the GRASS database

In this breakdown the first 2 steps would be specific to the format and
would result in a raw database, probably resident in temp files.  The last
three steps could be part of a generic import capability.  The generic
part of the process would start with the information stored in the temp
files and result in a final imported database.

In the case of a DXF file, this is OK, but the same procedure applied to a
format that included topology would result in the loss of the original
topology.  I presumed that a topology would be rebuilt with a new
v.support.

David, is an "arc" in your usage similar to a polyline?  I found the term
initially a little confusing.  Autocad uses the same term to refer to a
section of a circle specified with a center, a radius and beginning and
ending azimuths.


Roger Miller