The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
SRTM DEM does not significantly distort the terrain as 
represented by the 90-m SRTM DEM. This can also be seen 
in Figure 6 (f) which shows that the hydrographic network 
fits well with the contour maps derived from the SRTM 
DEM using the three methods described above. Again, this 
is a strong indication that further processing of the 90-m 
SRTM elevation data is required to ensure good 
cartographic quality of the derived contour maps for 
1:25,000 topographic mapping. 
5. CONCLUSIONS 
This study investigated the accuracy of contour interpolation 
from SRTM and existing 1:50,000 topographical maps. The 
various processing tasks executed were based on the 90-m 
resolution CGIAR-CSI SRTM elevation data, a 1:50,000 
topographic map of the test site and GPS readings acquired 
during a field truthing mission executed within the framework 
of an on-going state-wide 1:25,000 topographic mapping of 
Ondo State, in the South Western part of Nigeria. The 
topographic mapping project was originally executed using 
existing 1:50,000 topographic maps of the area as the base for 
extracting both planimetric and altimetric data. This study was 
therefore conducted to compare the accuracy of contours 
interpolated from the two sources in order to determine their 
suitability for deployment in the 1:25,000 topographical 
mapping. The following findings were made from this study: 
(1) both SRTM elevation data and elevation data from 
existing 1:50,000 topographic maps can be used to create 
a good representation of the terrain, judging from their 
high positive correlation with the more accurate GPS 
height data of points within the same site; 
(2) the 90-m resolution SRTM DEM manifests artefacts and a 
prior processing of the data is recommended to achieve 
cartographic quality good for 1:25,000 topographical 
mapping. 
These findings therefore indicate that 90-m resolution SRTM 
elevation data can be used as a substitute for existing 1:50,000 
topographic maps with the caveat that the former be processed 
prior to topographic information extraction for 1:25,000 
topographical mapping. To satisfy this requirement, the 
methodological approach (based on point interpolation using 
the moving surface function) proposed and tested in this study 
can be adopted. 
The analysis of the results emanating from the methodological 
approach proposed in this study for processing the SRTM data 
prior to contour interpolation was basically qualitative. In order 
to better conduct the analysis and assessment of the accuracies 
of the derivatives from the two elevation data sources used in 
this study, the applicability of other processing techniques 
proposed in the literature (such as wavelet transforms and 
Kalman filtering) in this context need to be further investigated. 
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