2. METHOD 
   
The statistical properties of the terrain surface are described by an "energy 
spectrum" compiled from measurements of profiles. The undulations of the ter- 
rain surface are plotted as function of the wavelengths of the undulations. 
The numerical values are derived by a Fourier transformation of the heights 
in the profile, and are shown in a double logarithmic graph. The capability. 
of this approach is only to show the stochastic undulations of the terrain. 
Surface features such as break lines cannot be described by this method and 
terrain types with many break lines have therefore been omitted. Non-random 
features such as a gradual slope or a bend in the profile must be removed 
as a trend from the height measurements before the Fourier transformation. 
In order to express the total energy of the profile, the Fourier coeffici- 
ents must be multiplied by the length of the profile. (Frederiksen and Ja- 
cobi, 1982). 
The spectra from the individual profiles are first smoothed, and the resul- 
ting spectrum is computed as an average of several profile spectra from the 
same area, as described by Frederiksen (1981). Some participants have mea- 
sured 2, some 3 and other 4 profiles in each area. 
Lac? (M3) 
  
105 
  
10% A 
  
  
  
Fig. 1. Energy spectra from: 
  
  
a. Drivdalen 
| —.—.—  Bohuslän 
| —ÁÜppland 
  
  
Each spectrum is estimated from 4 
rofiles measured in large scale 
p 
photographs in an analytical plot- 
107? ter by E. The sampling interval 
10° 10? 10! 109 T ; 
  
  
  
  
  
c 
was 5 meters in Drivdalen and 2.5 
WAVELENGTH (M) meters in the other landscapes.