I thank you for your quick answer. Maybe either my
answer wasn't clear or I did not explain in detail the
problem. I apologise also for my bad english.
I'm curious: how accurate are your elevation
measurements and how accurate
are horizontal positions?
Of course there are errors. I cannot quantify 'cuase I
didn't develop that measurements by myself.
The reason I ask is because most requirements for
"precisely localized"
models are based on a lack of understanding of
natural systems. If your
sampling points are 50 m apart, what do you mean by
"precisely localized"?
What I mean with the term "precisely" is that I wish
calculate the elevations of the eight neibourgh point
of each sample point measurement, just to calculate
terrain attributes with different grid size, even
greater than 50 m. I believe that in this way I could
avoid introducing another source of error. If I used
directly the output maps (slope, aspect, etc.) of
s.surf.tps, for example, the results may be non
representative of the grid cell centered "with a
certain casual error" on each of my 100 points not
being just in the middle of the grid cell.
The other consideration is the interpolation
algorithm used to build the 3D
surface from the point data. Several interpolators
have been developed. The
one most commonly used with elevation data assumes
unevenly-spaced
measurements and (by design) overshoots the high
points and undershoots the
low points. The reason for this is that almost never
do you have
measurements at the actual local maxima and minima.
Yes, I knew about that, errors are unavoidable. In
this moment I'm only a user of this kind of programs.
Besides, my point are along a trasect situated along
an hillslope, so there aren't peacke or valleys.
What I'm trying to say (in a round-about,
long-winded way) is that any DEM
you build is an estimate of the real world, and it
contains varying amounts
of inaccuracies. The only way you'll get "precisely
localized" models is to
have a survey crew place a number of markers at
known locations on the
ground, have the site flown by a skilled aerial
photography company, then
pay a photogrammetist big money to produce an
accurate DEM for you.
I thank you for your words, you've been very kind to
answer to my simple question, but I just want to know
if there are some grass programs that could manipulate
site list files in the same way of r.mapcalc. In this
way i should be able to solve my problem.
=====
Mario Palladino
DIAAT - University of Neaples
Via Università 100 - Portici - Napoli - Italy
tel/fax +39-081-7755341
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