The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
wide-range of applications and this development has been 
identified by professionals in the geo-information arena as a 
significant landmark that will tremendously revolutionize 
medium-scale topographic mapping. The near-global SRTM 
digital elevation model (DEM) product was processed and 
compiled at a resolution of 90m by the Consultative Group for 
International Agriculture Research Consortium for Spatial 
Information (CGIAR-CSI) and hosted on a Web portal for free 
public access and download. Although this product presents 
attractive promise for terrain analysis for an impressively wide 
range of applications, several researchers have proposed a 
thorough evaluation of its vertical accuracy. For instance 
Gorokhovich and Voustianiouk (2006) in their study to assess 
the accuracy of the SRTM-based elevation data noted that an 
overall assessment of the accuracy of the product requires 
additional regional studies involving ground truth control and 
accuracy verification methods with higher level of precision. 
Both the vertical accuracy of the product and its applicability in 
different contexts have been extensively investigated in many 
studies (Koch, A. and Lohmann, P., 2000; Miliaresis, G. and 
Paraschou, C. V. E., 2005: Kocak G. et al, 2005: Koch, A. et al, 
2005, Kleusberg, A. and Klaedtke, H. G., 1999). The major 
objectives of these studies have been to: 
• determine the absolute and relative vertical accuracies 
of the SRTM digital elevation data for defined study 
sites; 
• determine the relationship between SRTM vertical 
errors and certain topographic derivatives from the 
SRTM DEM such as slope and aspect; 
• evaluate the impact of spatial structure on the 
accuracy of contour maps derived from the SRTM 
DEM; 
• investigate the effects of geostatistical processing 
models on the SRTM DEM derivatives. 
The enormous attention directed towards regional-level 
assessment of SRTM vertical accuracy suggests that extracting 
topographical data by contour interpolation from SRTM 
elevation data necessitates accuracy assessment of the 
interpolation result to ascertain its suitability for digital 
topographical mapping especially at a scale of 1/25,000. In this 
paper, we present a framework for accuracy assessment of 
interpolating contours from 1:50,000 topographical maps and 
SRTM height data for topographical mapping at the scale of 
1:25,000. Quantitative statistical and geostatistical tests were 
performed on the two spatial data sources for different terrain 
configurations and contexts to determine their suitability for 
topographical mapping in different scenarios. In particular, this 
study consisted of: 
(1) measuring the vertical accuracy of the DEM derived 
from the 1:50,000 topographic map and that of the 
90-m resolution CGIAR-CSI SRTM digital elevation 
data against higher precision GPS measurements 
within the same site; 
(2) interpolating digital elevation models from an 
existing topographic map covering the same area and 
comparing measurements from the two sources; 
(3) implementing a processing strategy to minimize 
errors emanating from contour interpolation using 
SRTM data as a base. 
Using an on-going 1:25,000 state-wide topographical mapping 
project undertaken by one of the State Governments of Nigeria 
as a case study, we demonstrate the applicability of the 
proposed contour interpolation accuracy assessment model for 
arriving at the best strategy for the topographical mapping 
process. The rest of the paper is organised as follows. Section 2 
describes the site chosen for this study. Section 3 is dedicated to 
a description of the materials and the methodology adopted for 
the study. Section 4 presents a discussion of the results of the 
GIS analysis and geostatistical processing methods adopted 
while Section 5 concludes the paper. 
2. STUDY SITE 
The site chosen for this study lies between Latitudes 5°30’ E 
and 5° 45’ E and Longitudes 7° 15’N and 7°30’N. The site is 
situated in Ondo State, a state in the South-Western part of 
Nigeria. Figure 1 shows a digital elevation model (DEM) of the 
site presented as a color-coded image. The chosen site 
corresponds to the area covered by one map sheet at the scale of 
1/50,000 (27.8Rm X 27.5Km). The site covers low-level, mid 
level and high-altitude terrain with elevations ranging from 
182m to 594m above mean sea level. The mean elevation of the 
site is 284m above mean sea level. As depicted in Figure 1, the 
South-Eastern part of the area is generally low-lying with 
elevations ranging from 182m above mean sea level to 285m 
above mean sea level. The North-Eastern part is characterized 
by rugged terrain interspersed with hills having elevations 
ranging from 490m to 594m above mean sea level. The rest of 
the area is generally at mid-level altitudes with elevations 
ranging from 285m to 388m above mean sea level. The entire 
area is well-drained with a good network of rivers and streams 
all flowing Southwards. The predominant vegetation in this area 
is of tropical rain forest type with dense canopy cover in some 
areas and low, widely separated trees in others. The hilly areas 
present exposed rock surfaces in some areas and low grasses 
and shrubs in others. This site was deliberately chosen by 
reason of the fact that it presents different topographic 
conditions (from low to high terrain characteristics) for the 
purpose of conducting statistical and geostatistical analyses 
presented in this study. 
£t «vatio» Voily Ar»« 
Figure 1. The study site as oolor-coded Digital Elevation Model 
3. DATA, MATERIALS AND METHODS 
3.1 Source Data 
The accuracy tests conducted in this study employed three 
major sources of spatial data (90-m resolution CGIAR-CSI 
SRTM digital elevation data, 1/50,000 topographical map of the 
study site and GPS point data randomly distributed over the 
study site). 
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