The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Voi. XXXVII. Part B7. Beijing 2008 
1352 
study site. The degree of nearness of the two surfaces to reality 
can also be seen from the high F-statistic values of the two data 
sets (0.767496217 for the SRTM elevation data and 0.990632 
for the topo DEM data). These values indicate that the topo 
DEM is a more accurate surface than the SRTM DEM, a 
finding that informed our using the topo DEM as a reference 
surface for the analyses of the SRTM DEM throughout the rest 
of the study. 
Figure 6. Contours interpolated from SRTM DEM 
(a) 5-m vertical interval contours directly interpolated 
from the 90-m SRTM DEM showing artefacts in circles 
(b) 5-m vertical interval contours interpolated from the 90- 
m DEM derived from the 1:50,000 topographic map by 
moving surface interpolation 
(c) 5-m vertical interval contours interpolated from the 
SRTM DEM re-sampled to 45m by cubic interpolation 
(d) 5-m vertical interval contours interpolated from the 45- 
m DEM derived from the 1:50,000 topographic map by 
moving surface interpolation 
(e) Super-imposition of the maps from (a) - (d) above 
(f) Super-imposition of the maps from (a) - (d) above and 
the hydrographic network 
4. RESULTS AND DISCUSSION 
The computed TIN-based SRTM and topo DEMs overlaid with 
the hydrographic network covering the study site (Figure 3(a), 
Figure 3(b), Figure 4(a) and Figure 4(b)) revealed striking 
characteristics of the two surface representations. The overlay 
showed that the hydrographic network matched the two 
surfaces well, with the rivers passing through the water courses 
in the two DEMs. This is a particularly interesting result noting 
that the two surfaces were derived from two disparate sources 
(1:50,000 topographic map and SRTM data) with the 
hydrographic network layer coming from the topographic map 
data source. The terrain profile presented in Figure 5 further 
revealed that the two surfaces are generally close, implying that 
SRTM elevation data can be used as a substitute for the 
topographic map elevation data for 1:25,000 topographical 
mapping. 
The results of the various tests conducted to assess the 
cartographic accuracy of the CGIAR-CSI SRTM elevation data 
are as presented in Figures 6(a) - (f). The tests essentially 
involved direct interpolation of 5-m vertical interval contours 
from the SRTM DEM, recreation of the SRTM surface by re 
sampling and point interpolation and super-imposition of the 
various maps for visual interpretation. Based on a visual 
interpretation of the results, the following facts can be deduced 
about the CGIAR-CSI SRTM: 
(1) Direct interpolation of contours from the 90-m SRTM DEM 
without further processing produces artefacts in the form of 
incomplete contour lines, self-intersecting contour lines and 
contour lines intersecting other contour lines having 
different contour values (see Figure 6 (a) showing artefacts 
in encircled areas of the map). This result is a strong 
indication that contour maps directly derived from the 
SRTM DEM are generally of low cartographic quality and 
are not recommended for deployment in 1:25,000 
topographic mapping without prior processing. 
The various tests conducted in this study to assess the accuracy 
of contour interpolation from the 1:50,000 topographic map and 
the SRTM elevation data to investigate their suitability for 
1/25,000 topographic mapping revealed striking characteristics 
about the data sets. The statistical computation for the absolute 
vertical accuracy of CGIAR-CSI SRTM elevation data for our 
study site gave a value of ± 7.748m. This statistic indicated 
that, for our study area, the 90-m CGIAR-CSI elevation data 
featured a much greater absolute vertical accuracy than the 
absolute vertical accuracy value of ± 16m published in the 
SRTM data specification. The statistical tests also revealed that 
the absolute vertical accuracy ( ± 3.926m) of the digital 
elevation data derived from the 1:50,000 topographic map 
covering our study site was much higher than that of the 
CGIAR-CSI SRTM DEM. The graphic plots of the GPS 
elevations against the SRTM and topo DEM (Figure 2(a) and 
Figure 2(b)) revealed that the two data sets showed strong 
positive correlations with the GPS data. This suggests that the 
two surfaces are significantly close to the real surface for our 
(2) Processing the SRTM DEM before interpolating contours 
from it produces results with significantly better 
cartographic quality than direct interpolation. This 
observation can be clearly seen in Figures 6(b) - (d) which 
respectively show the results of moving surface 
interpolation (90m) of the derived point raster set, the 45-m 
re-sampling of the 90-m resolution SRTM DEM and the 
moving surface interpolation (45 m) of the derived point 
raster set. In all these output contour maps, it can be seen 
that the artefacts present in the directly interpolated contour 
map are clearly absent, giving 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. 
(3) The forms of the contour lines emanating from processed 
and unprocessed SRTM DEM are essentially the same. This 
can be seen from Figure 6(e) depicting a super-imposition 
of the contour maps from the three cases cited above. This 
result is a good indication that further processing of the