In: Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
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3.3 Validation of the terrain roughness parameters 
In a first processing step the derived terrain roughness layers 
(standard deviation of detrended terrain points, mean echo 
widths of terrain points) are compared. A linear regression from 
both roughness images, according to the formula y = a + b*x, 
where x and y represent raster maps was calculated. In addition 
to the entire image, subareas were selected, each representing 
an individual land cover type (see subareas in Fig. 1) i.e. grass 
(3), bushes (2), young (1) and old (4) forest for calculating the 
linear regression. Finally, a plausibility check was done by field 
investigations. Furthermore, the effect of varying footprint sizes 
caused by multiple echoes on the derived echo widths is 
studied. To ensure, that only echoes with footprints sizes, which 
are in a similar range, are used, only single echoes (i.e. 
extended targets) that are below the defined height thresholds 
were selected. The selected single echoes are used to generate 
additional roughness layers. 
4. RESULTS AND DISCUSSION 
Fig. 5 shows the standard deviation of the detrended terrain 
points as well as mean echo widths of using all echoes. High 
standard derivations (bright areas) indicate a high terrain 
roughness. Large echo widths (bright areas) are caused by a 
certain vertical distribution of scatterers within a laser shot 
footprint and hence large echo widths indicate high terrain 
roughness. The visual comparison of the derived roughness 
layers (Fig. 5) clearly shows similar spatial patterns of areas 
with high values of terrain roughness. It can also be shown that 
this similarity is apparent for all applied height thresholds. The 
highest correlation coefficients (R-0.62 - 0.63) are available for 
the roughness layers derived from echoes with heights (dz) less 
than 0.25 m and 0.50 m, respectively. The linear regression 
between the standard deviation of the detrended terrain points 
and the mean echo widths derived from the extended targets 
show a slight decrease of the correlation coefficients e.g. for 
echoes with dz < 0.5 m from R=0.63 to R=0.58. 
Figure 5. Roughness layers derived from FWF-ALS data. Left: Mean standard deviation per raster cell of detrended terrain points for 
different height thresholds. Right: Mean echo width per raster cell calculated from terrain and near terrain points respectively.