RE: DEMs, TINs, orthos, draping

["Crispen, Bob" <Robert.Crispen@HSV.Boeing.com>]

> Tapani Sarjakoski[SMTP:Tapani.Sarjakoski@fgi.fi] sez:
> 
> A. Digital elevation models (DEMs)
> 
> There are two main approaches for representing elevation 
> of the ground in digital form: a) regular grids and b) 
> irregular triangulation networks (TINs). 
[much good stuff snipped]

Let me ask a couple of very simpleminded
questions if y'all don't mind (again!).

If you've got a series of points that are not on
a regularly spaced grid and want to end up with
a regularly spaced grid (in VRML terms, going
from an IndexedFaceSet to an ElevationGrid),
am I correct in concluding that you create the grid
using bilinear interpolation even though you're
virtually certain to miss the high and low points
in the terrain (I'm assuming that the elevation
data you're starting with has been sampled at
"interesting" points).

What algorithms do you use for thinning?  If
I want a lower LOD, should I simply pick, say,
every tenth point, or is there a better way that
most folks use?

What about going the other way -- from elevation
data on a regular grid to thinned elevation data
with irregularly spaced points (in VRML, from
ElevationGrid to IndexedFaceSet)?  Is there a
conventional way to do this?  I think graphics
folks would use a mesh decimation algorithm
here.

Bob Crispen
bob.crispen@boeing.com