The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
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ActiveX GIS control) to perform this operation. The result was
a dBase table which was imported into Microsoft Excel
environment to perform the statistical analysis. The results of
the computation are summarized in Table 1 below. The table
shows that the absolute vertical accuracy of the CGIAR-CSI
SRTM elevation data for our study site is ± 7.748m and a
mean difference in elevation between the two data sets is
3.539m. Figure 2(a) depicts a graphic plot of the GPS
elevations against the SRTM elevations. The coefficient of
correlation and F-statistic between the two data sets were
respectively +0.993094578 and 0.767496217. These results
indicate a strong positive correlation between the two data sets.
Since this study also involved determining the vertical accuracy
of the DEM derived from the 1:50,000 topographic map, this
statistic was computed by applying the same method described
for the SRTM data. The results of the statistical computation
are presented in Table 2. The results show that the absolute
vertical accuracy of the topo DEM for our test site was
± 3.926 and a mean value of-0.151m. A graphic plot of the
GPS elevations against the topo elevations is as depicted in
Figure 2(b). The coefficient of correlation for the data set was
+0.998253 with an F-statistic of 0.990632.
Statistical
Measure
SRTM
Elevation (m)
GPS
Elevation(m)
Ah
(SRTM-GPS)fm)
Minimum
190.000
184.324
-19.996
Maximum
489.000
507.168
17.162
Mean
327.308
323.768
3.539
Std Dev.
± 63.425
± 65.101
± 7.748
Count
130
130
130
Table 1: Statistical analysis of SRTM and GPS elevation data
Statistical
Measure
Topo
Elevation (m)
GPS
Elevation(m)
Ah
(Topo-GPS)(m)
Minimum
182.88
184.324
-18.985
Maximum
502.92
507.168
18.542
Mean
326.568
323.768
-0.151
Std Dev
± 66.38
± 65.101
± 3.926
Count
139
139
139
Table 2 Statistical analysis of Topo and GPS elevation data
Graph of GPS Elevations against SRTM Elevations
Graph of GPS Elevations against Topo Elevations
Figure 2. Graph of GPS elevations against
(a) SRTM elevations (b) Topo elevations
3.3.4 Comparison of the SRTM DEM and the
Topographical DEM surfaces: For contours interpolated from
the SRTM DEM to be deployed in 1/25,000 topographic
mapping, the accuracy with which the DEM represents the
surface morphology must be tested and proved to be of a
sufficiently high degree. In this study, we employed two
methods to test the accuracy of the SRTM data for good terrain
representation. The methods involved the following
(1) interpolating a DEM from the topographical map
with the same spatial resolution as the SRTM DEM
and visually comparing the two surfaces;
(2) generating profiles along defined transects on the
two surfaces and visually comparing the plots.
The above operations required that the two DEMs must have
the same spatial resolution, georeference and spatial extent. To
satisfy this requirement, the output DEM from the
topographical map with a resolution of 90m was trimmed to the
size of the SRTM DEM. Both grid-based DEMs were converted
into TIN-based DEMs in the ArcGIS 9.2 environment and
overlaid with the hydrographic layer. The results of this
operation are as shown in Figure 3 (a) and Figure 3(b). Figure
4(a) and Figure 4(b) show the perspective views of the two
DEMs.
(a) (b)
Figure 3 TIN-based DEMs with the hydrographic network
superimposed (a) Derived from 1/50,000 Topo Map
(b) Derived from SRTM Grid
(a) (b)
Figure 4. Perspective views of derived DEMs with
hydrographic network superimposed
(a) Topo DEM (b) SRTM DEM
Further analysis of the SRTM surface involved a comparison of
height profiles on the two surfaces. Two transects running from
SW to NE and NW to SE were created on the two DEMs. To
extract the elevations for the profile plot, we implemented a
small Visual Basic 6.0 program to project the transects onto the
DEM surfaces and to extract elevations from the two surfaces at
regular intervals. The result of the computer run of the program
was a dBase table containing the elevations along the transects
on the two surfaces and the cumulative distances from the
starting points of the profiles. The table was imported into