1347 
ACCURACY ASSESSMENT OF CONTOUR INTERPOLATION FROM 1:50,000 
TOPOGRAPHICAL MAPS AND SRTM DATA FOR 1:25,000 TOPOGRAPHICAL 
MAPPING 
A. P. Ozah a ' *, O. Kufoniyi b 
Régional Centre for Training in Aerospace Surveys (RECTAS) 
PMB 5545, Off Road 1, OAU Campus, Ile-Ife, NIGERIA - 
a azukaozah@yahoo.com;b(kufoniyi@rectas.org, jidekufoniyi@yahoo.com) 
Commision VII, WG VII/7 
KEY WORDS: Topographic mapping, Map revision, Shuttle Radar Topographic Mission data, Contour interpolation, Contour 
accuracy assessment 
ABSTRACT 
One major problem engendered by poverty in less-developed countries is that of inability of such countries to carry out fresh 
medium-scale topographical mapping at regular time intervals using expensive but accurate topographical mapping techniques. For 
such countries, an alternative which has proved to be cost-effective and efficient over the years is topographical map revision using 
existing topographical maps, satellite imageries and other free spatial data sources. However, extracting contours from existing 
topographical maps for inclusion in a new digital topographical map, often at a larger scale, is not usually a straight-forward process 
due to differences in map units and contour intervals between the existing base maps and the new map. Although free spatial data 
sources such as the Shuttle Radar Topographic Mission (SRTM) digital data provide excellent base data for extracting height data 
for topographic mapping, such data sets need to be adequately evaluated and subjected to further processing before extracting 
contours needed for topographical mapping. Extracting topographical data by contour interpolation from existing topographical 
maps and SRTM data therefore necessitates accuracy assessment of the interpolation result to ascertain its suitability for 
topographical mapping. This paper presents 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. Accuracy tests of contours 
interpolated from the two sources are performed for different terrain configurations and contexts to determine their suitability for 
topographical mapping in different scenarios. Using an on-going 1:25,000 topographical mapping project as a case study, the use of 
this contour interpolation accuracy assessment model for arriving at the best strategy for the mapping is also presented. 
1. INTRODUCTION 
Topography is basic to many earth surface processes and thus 
finds applications in ecology, hydrology, security, agriculture, 
climatology, geology, pedology, geomorphology and a host of 
other domains and constitutes the basis for explaining processes 
and predicting them through the process of modelling. The 
tremendous role of topographic mapping in national 
development continues to receive recognition by national, state 
and local governments the world over. The importance of 
topographic mapping as a national project is therefore growing 
and accurate topographic maps as its major products are 
considered as indispensable components of national geospatial 
data infrastructure. In countries with developed and stable 
economies, a clearly articulated road map is usually made for 
regular, fresh topographic mapping using current data and the 
state-of-the-art mapping technologies. In less-developed 
countries however, the problem of poverty and inadequate 
technical capacities in the area of geo-information production 
and management culminate in the inability of such countries to 
carry out fresh topographical mapping at regular time intervals 
using expensive but accurate topographical mapping techniques. 
For such countries, an alternative and cost-effective strategy 
over the years has been topographical map revision using 
existing topographical maps, satellite imageries and other 
readily available spatial data sources. Unfortunately, the 
* Corresponding author. 
process of extracting topographical information (contours) from 
existing topographical maps and integrating same in a new 
digital topographical map, often at a larger scale, is usually a 
lengthy and time-consuming process due to differences in map 
units and contour intervals between the existing base maps and 
the new map. Moreover, most of the topographical map sheets 
to be used as base maps for a revision exercise are either 
missing or, where they exist, are very old and suffer from 
severe distortion. This method of topographic mapping 
therefore turns out to be error-prone, time-consuming and 
highly-demanding in manpower resources. 
A recent development representing a tremendous forward leap 
in remote sensing technology that will significantly eliminate 
some of the lacunae associated with topographic map revision 
from existing topographical maps is the launching of the Shuttle 
Radar Topography Mission (SRTM) in February, 2000. Using 
the Synthetic Aperture Radar (SAR) interferometry to produce 
the first near-global high resolution digital elevation model 
(DEM) of the Earth, SRTM has created an unparalleled set of 
global elevations that is freely available for modelling, mapping 
and environmental applications (Gorokhovich and Voustianiouk, 
2006). The global availability (about 80% of the Earth surface, 
covering land masses between 60°N and 56°S) makes it the 
most widely-used set of baseline elevation information for a