PERFORMANCE AND APPLICATIONS OF ORTHOPHOTOMAPS 
by 
J. Visser M. C. van Wijk 
ITC, Netherlands National Research Council of Canada 
L. van Zuylen R. Mullen 
ITC, Netherlands U. S. Geological Survey, USA 
Abstract 
Hypsographic information may be considered indispensable to most users 
of orthophotomaps. Current methods of obtaining this information from 
instruments in which profiling is done manually as well as those in- 
struments in which the profiles are produced automatically through the 
use of image correlators are discussed. The deficiencies of height 
information derived from dropped lines, especially at the larger map 
Scales are noted. Proposals are presented for improved systems of ob- 
taining necessary height information. 
Three dimensional interpretation of the orthophoto image, combined 
with the possibility of extracting precise vertical information on any 
point or feature of the terrain is offered by the stereoscopic ortho- 
photo technique developed at the National Research Council of Canada. 
The techniques and equipment for compilation from stereoscopic ortho- 
photos are presented. Vile these orthophotos are produced on equipment 
still considered to be experimental, which affects speed of production 
and accuracy of the final results, some results obtained from large 
Scale mapping experiments are encouraging. s 
'A review is made of the literature on cartographic treatments for 
orthophotomaps. Results of a study of the transfer of resolution in 
the production steps of orthophotos is given with emphasis on repro- 
duction techniques involved for the production of small numbers of 
map copies. 
The applications of orthophotoproducts as the map base used in pro- 
duction of other map types or for special map uses such as urban maps, 
coastal maps and charts and for airport obstruction charts are dis- 
cussed along with the use of orthophotomaps for the state or national 
map series at both large and small scales. 
Hypsographic Information on Orthophotomaps 
For the great majority of the users of orthophotomaps - either large 
scale such as 1:1,000 orthophotomaps for | urban development (Ref.: 
Ackermann 1969) or small scales such as 1:50,000 annotated orthophoto 
map series covering the vast desert areas of Saudi Arabia (Ref.: 
Waldhausl 1971) - hypsographic information is indispensable. 
The accuracy required of this hypsographic information (i.e., of the 
overprinted contours and spot heights) increases, in a relative sense, 
with increasing map-scale. It is usually the requirements for semantic 
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