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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
Where a = the standard deviation of an average difference 
with the weight equal to 1, and 
g = weight (Equation 5). 
Calculated average differences for every parcel were 
subsequently analysed as a function of the land cover. 
3. RESULTS 
The vegetation impenetrability and anthropogenic obstructions 
found above the surface of the Earth introduce asymmetry into 
the histogram of elevation differences (SRTM minus ‘bald- 
Earth’ reference elevations) (Heipke, et al., 2002). Histograms 
in Figure 5, 6, and 7 were developed for vegetation 
impenetrabilities (Equation 2). The mean differences for 
SRTM.C/X minus the reference elevation are 6.62 m and 2.53 
m, respectively. Hence, the mean difference for SRTM.C minus 
SRTM.X is about 4 m. 
The histograms in Figures 5 and 6 do not follow the Gaussian- 
fitting curve. This is because they represent a compound 
probability distribution function of two stochastic processes: 
one is the ‘bald-Earth’ difference between SRTM and reference 
DTM, which follows the Laplace distribution; the second one is 
created as the same difference but considered over vegetated 
areas—it follows the Gaussian distribution. The Gaussian 
probability density function is given by the well-known formula: 
f(x\m,<j)=—= t 
erf 2 n 
(7) 
where 
m = mean value, and 
ct = standard deviation. 
0.18 
C - <R> 
Mean = 6.62m 
Median = 4.1m 
0.15 
STD = 11.01m 
0.12 
> 
o 
CD 
D 
f 
LL 
0.06 
0.03 
The Laplace probability density function is given by (Norton, 
1984): 
*-/'P 
where 
f(x\p,b)=—e [ 
p = location parameter, 
b = scale parameter. 
(8) 
For N independent and identically distributed differences d h 
d 2 , ..., d N , estimator of p - Jj is the median of differences, and 
the estimator of b - b can be calculated using the maximal 
likelihood estimator from: 
-20 -10 0 10 20 30 40 50 
Difference (m) 
HP--* 
(9) 
Figure 5. Histogram of buffer-based elevation differences C- 
band minus reference elevation. 
0.18 
X-<R> 
Mean = 2.53m 
Median = 0.4m 
STD = 6.74m 
0.15 
0.12 
0.06 
0.03 
-20 -10 0 10 20 30 40 50 
Difference (m) 
Figure 6. Histogram of buffer-based elevation differences X- 
band minus reference elevation. 
0.2 
Buffers = 53,190 
Mean = 4.0m 
Median = 3.9m 
C-X 
Difference (m) 
Figure 7. Histogram of differences C-band minus X-band. 
Laplace fitting curve was drawn for p = 3.9 m and b = 4.96 m. 
Gaussian fitting curve was drawn for m - 4.0 m and o = ±7.8 m.