[GRASS-user] TIN and B/T correction in grass...

[Hamish]

Hamish ha scritto:
> > Luca Penasa wrote:
> > > Is there a way for apply a bridge/tunnel TIN correction in GRASS?
> > > Is there any other similar TIN correction yet implemented in GRASS
> > > GIS? Anybody know if this function is only implemented in IDRISI?

Hamish ha scritto:
> > can you explain what this function does exactly?

Luca Penasa wrote:
> [Manual]
> "Contour lines at the top of a hill [...]
> Because all three of the points for all the triangles have the same
> elevation, the top of the hill is perfectly flat in the TIN model."

(see hint in r.surf.contour manual about importance of including trig and
ridge points in surface generation, if available)

> [Manual]
> "Our experience with actual terrain tells us that the true surface is
> probably not flat, but rather rises above the TIN facets. The edges of
> the TIN facets that lie below the true surface in this case are
> examples of what are called "tunnel edges". [...] A tunnel edge is any
> triangle edge that lies below the true surface.
> Similarly, if the contours of Figure 11-4a represented a valley bottom
> or depression, the TIN facets of 10-4b would describe a flat surface
> that is higher than the true surface. The edges of the TIN facets that
> lie above the true surface would then be termed "bridge edges"."


Do you have a TIN you would like to fix in GRASS? (original or rasterized?)
Or you are wondering how GRASS deals with the bad side effects of TINs?

To answer the first: each triangle confluence/vertex is a x,y,z data point.
Together these describe all the available actual data in the model, the
rest of the TIN is linear interpolation with some simple rules.
What you need to do in GRASS is load or convert the TIN into its
fundamental x,y,z data points, then use one of GRASS's more sophisticated
surface generation modules to create a new surface.  i.e. use v.surf.rst
to generate a more realistic DEM with peaks and valleys than the simple
TIN input describes.

To answer the second: GRASS doesn't really "do" TINs, for (AFAICT) the
simple reason that it doesn't have to resort to that. It has a very good
and mature raster engine with much better surface generation tools. IMHO
TIN is a hold-over technique for faux-raster surfaces from other GISs
which have been historically strong with vectors but weak with raster
data. (I would not count IDRISI among those, I speak more of Arc/Info,
but...) You can search the mailing list archives for my previous diatribes
on the subject. Of course all are free to work with whatever tools they
prefer, there is a little about GRASS+TINs here:
  http://grass.osgeo.org/wiki/Triangle_Mesh

Unfortunately AFAICT mediawiki's search engine does not consider three
letter words so it is hard to guess if there is any more content. (if not
there will be in the mailing list archives)

So my advice is to spend some time with v.surf.* and r.surf.* (including
r.surf.nnbathy from wiki addons) and enjoy the things that GRASS does well.


2c,
Hamish