B3. Istanbul 2004
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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
A local discrepancy between simulated and observed ERS
heights is the sign of a potential anomaly in the DEM.
A systematic shift between both height is the sign of a
systematic shift error on DEM.
6. ERS ERROR ANALYSIS ON NON-FLAT TERRAIN.
We investigated the ERS elevation data error on rougher
terrain, using a 30-meter digital elevation model considered as a
reference. Two different study areas with different relief
characteristics were selected. We derived parameters from this
DEM. Errors between ERS data and DEM and also between
the simulated responses obtained with the DEM were
calculated. Finally, their correlation with the terrain parameters
were analysed.
6.1 Study areas description
The first geographical area, located at the south-west of France,
includes landforms ranging from extensive floodplains to low
relief foothills, and high relief, long mountains slopes to the
east. The other area, located in the North, is less rough but
contained larger urban area, which can distort the altimeter
response. The field areas are approximately 150 km?.
6.2 Elevation data
ERS elevation data : we collected all data obtained after the
retracking step in both areas. There were 5679 and 8634
elevation points over each area.
30-m DEM : the DEM is a level 1 data (DTEDI) acquired by
photogrammetric method from remote sensing images such as
Spot, or by contour digitising from existing 25,000 scale maps.
A root mean square errors (RMSE) is provided to express its
quality . The RMSE is reported as 30 m for horizontal
coordinates, and 5 m for height, relative to the WGS 84 datum.
We extracted elevation at each ERS elevation positions using a
bilinear interpolation method.
ERS simulated elevation data : at each ERS elevation position,
we use the DEM to obtain a simulated ERS height based on the
method described in previous section .
160 ———
i n (À {7 DEM |
Wi + ERS |
155} : " A
n
Vis
| ; Ff +
7 +
150 due *
Ta * i
y u * #
; "nf.
i, ul Lan
~ 145} P mu EE
E ra ^
8 7
© ue
= 140}- i a
i »
| p
| A
| : | x
135} M^ Hur
SU
Qn Mau
1304. ^
UM Mf
Bu
125 i 1
0 10 20 40 50 60
distance (km)
Figure 6. Height profiles from DEM and ERS altimeter
63 Exploratory data analysis
63.1 Errors statistical distribution
Fig 7 shows the different data sets with their histograms. They
seem similar for the three data sets. On the north area, they
show a roughly normal distribution of the elevations. On the
second sites they are broader, showing the various relief. The
errors were calculated by subtracting the interpolated DEM
elevation and the simulated elevation from the ERS measured
elevation. The spatial distribution of these absolute error values
and their histograms are plotted in fig.8.
study area
2500 —m 2800p—————————À 2500 ——— —— — —
mean: 114 mean: 129 mean: 137
2000: std: 45 : 2000: sid: 31 2000 sid: 26
max :254 max :354 max :252
min 10 min :17 min :32
1500 - 1500 1500 :
1000 1000 1000
500 500 500
0 0t Ima 0 :
0 100 200 300 0 100 200 300 0 100 200 2300
elevation (m) elevation (m) elevation (m)
second study area
800 . , . 800 + 800 .
mean: 102 mean: 112 mean: 131
std: 57 std: e e std: 60
600: max 301 i i max : : max :322 i
min :0 600 min :-15 600 min :1 |
400+ 400: 400: i
200 200: 200:
0 0 = z
0 100 200 300 0 100 200 300 0 100 200 300
elevation (m) elevation (m) elevation (m)
Figure.7. Elevation histograms in both study areas.
first study area
7000; ——áÓ—— YOOÜ ree
| |
mean : 15 l mean : 11
6000 mem | 6000 mem
5000 maciTs | 5000 ue
min: 0 | min: 0 |
4000 | 4000 |
3000 | 3000 |
2000 | 2000 |
1000 | 1000 |
i {
0 ya 0 ec |
0 50 100 150 200 0 50 100 150 200
abs(DEM-ERS) (m) abs(SIM-ERS) (m)
seconc stucy erea
3000| i 3000 |
mean : 29 | mean : 22 |
2500 std: 29 { 2500 std: 28 |
i max:163 max:257 |
2000| min: 0 2000 min: 0 |
1500 : 1500 |
1000 1 1000 |
500 | 500
6 iad à ud c
0 50 . 100 150 200 250 0 50 100 150 200 250
abs(DEM-ERS) (m)
abs(SIM-ERS) (m)
Figure.8. Absolute errors, DEM and simulated ERS elevations
versus ERS measured elevation.
The histograms indicate that on average the altimeter gives a
coherent elevation value over the study areas. However, the
maximum absolute errors values show there are significant
differences in some areas. The distribution error is narrower,
particularly in the place with low roughness.
6.3.2 Terrain parameters influence.
To understand the altimeter behaviour, we derived some
parameters from the DEM reflecting the local topographic
roughness around each elevation position within a moving
window (a 20-cell or 10 km circle). The parameters are the
following :
- Pl and P2: the slope mean and standard deviation.
- P3 and P4: the mean and standard deviation of elevations.
The next table shows the coefficient for correlation between the
errors and the different parameters over both study area.