local elevation

[Rich Shepard]

On Thu, 16 Mar 2000, cicico messere wrote:

> 1)I have a site list with digitalized elevation data.
> 2)I have a site list of 100 sampling locations spaced
> 50 meters apart
> 
> What I want is a 3x3 matrix of elevations *centered* on each sampling
> location I need these figures to calculate precisely localized topographic
> attributes.

  I'm curious: how accurate are your elevation measurements and how accurate
are horizontal positions?

  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"?

  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.

  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.

Rich

Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
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