In: Wagner W„ Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Voi. XXXVIII, Part 7B 
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Figure 6. Shading of the standard deviation of the detrended terrain echoes (dz < 1.0 m overlaid with pictures from different features. 
This indicates that varying footprint sizes of single and multiple 
echoes have a negligible influence on the terrain roughness 
parameterization. For the young forest subarea (see Fig. 1, 
region 1) the correlation coefficients increases e.g. to 0.68 for 
echoes with dz<0.25 m. For all other subareas (see Fig. 1, 
region 2-4) the correlation coefficients decrease. However, as a 
decrease was not expected further statistics were calculated. For 
the grassland subarea the standard deviation of the mean echo 
width is 0.03 ns and for the old forest stand subarea the standard 
deviation is 0.02 ns. Also the standard deviation of the mean 
standard deviation per raster cell of detrended terrain points are 
small and are in the range of 0.4 to 1.9 centimeters. This means 
that there is only a constant shift between the two roughness 
layers. Furthermore, this indicates very homogeneous and 
smooth surfaces, which corresponds to the information derived 
during the field check. Moreover, this small standard deviations 
can be interpreted as measurement noise of the used FWF-ALS 
system. Even in the densely forested areas with FWF-ALS it is 
possible to acquire a sufficient amount of elevation 
measurements of the forest terrain surface. As shown in Fig. 6 
areas indicated by high roughness values are mainly covered by 
low vegetation (e.g. bushes) or exhibit a very dense distribution 
of tree stems, which also account for terrain roughness as 
defined in this study. High roughness values are also available 
at the borders of forest roads and tracks where commonly 
discontinuities (e.g. breaklines) are available. The results let 
assume that FWF-ALS echo width values are able to identify 
areas with high terrain roughness without the need of a highly 
detailed geometrical representation of the ground surface by 
acquiring very dense point clouds. However, there are still 
some uncertainties by using the echo width for terrain 
roughness parameterization. While pulse width estimates are 
relatively stable at high amplitudes, there is significant 
scattering at low amplitudes (Lin and Mills, 2010; Miicke, 
2008; Wagner et al., 2006). This needs to be taken into account 
in future studies. Furthermore, the comparison of the derived 
roughness layers has shown that small (i.e. in relation to the 
footprint size) terrain surface discontinuities i.e. breaklines 
which are not covered with vegetation or stems lying on the 
terrain are better represented in the geometry-based roughness 
layer than in the mean echo width (Fig. 7). This can be 
explained by the small laser footprints, and therefore, by the 
fact that within the small illuminated area only little vertical 
variations are available. 
5. CONCLUSION AND OUTLOOK 
In addition to the 3D-coordiantes, FWF-ALS delivers laser 
point attributes (e.g. signal amplitude, echo width) providing 
further quantities to characterize Earth surface properties. The 
presented analyses have shown that the FWF-ALS echo width 
derived roughness layer indicates areas with high roughness 
similarly to the geometric definition using very high laser point 
densities. Concluding the echo width can be used as a terrain 
roughness parameter even with low terrain point densities 
compared to the geometry-based computation.