COMMENTS 
On a previous occasion a similar accuracy estimation procedure, based 
on Fourier transformation of terrain profiles, has been introduced by 
the authors (Frederiksen, Jacobi and Justesen, 1978) and (Frederiksen, 
Jacobi and Kubik, 1984). 
Both the variogram estimation and the concept of terrain spectra have 
been applied to practical DEM planning purposes for various types of 
terrain. 
In all landforms the theoretical result, that the slope of the vario- 
gram is the slope of the terrain spectrum minus one 
8 = -(a-1) (8) 
has been confirmed, and consequently the two methods yield concordant 
accuracy estimates or similar optimal sample spacings for pregiven 
planning parameters. 
REFERENCES 
/1/ Berry, M.V. and Lewis: Proceeding of the Royal Society, London. 
(1980). 
/2/ Dixon and Maney: Introduction to Statistical Analysis, McGraw Hill, 
London (1969). 
/3/ Frederiksen, P., 0. Jacobi and K. Kubik: Measuring Terrain Roughness 
by Topological Dimension. Proceedings. Internat. 
Coll. on Mathematical Aspects of Digital Eleva- 
tion Models, Stockholm. (1983). 
/4/ Frederiksen, P., 0. Jacobi and J. Justesen: Fourier Transformation 
von Hohenbeobachtungen, ZfV, 103. (1978). 
/5/ Frederiksen, P., 0.Jacobi and K. Kubik: Accuracy Prediction for Digi- 
tal Elevation Models. XV Congress of ISPRS, Comm. 
III, Rio de Janeiro. (1984). 
/6/ Matheron, G.: The theory of regionalized variables and its appli- 
cations: Les Cahiers de Centre de Morphologie 
Mathematique, Ecole National Supérieure des Mines 
de Paris. (1971). 
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