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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