RESULTS OF THE INTERNATIONAL ORTHOPHOTO EXPERIMENT 1972-'76 
by 
Dr. T.J. Blachut and M.C. van Wijk 
Introduction 
As a result of an increased interest in the orthophoto tech- 
nique a variety of orthophoto printers has become available during recent 
years. In many cases these printers have been designed to be used in 
conjunction with existing photogrammetric plotting equipment; this is one 
of the reasons that operational modes, image transfer techniques and 
presentation of height data vary considerably for the different orthophoto 
systems. Some instruments, for example, operate strictly inan "on-line" 
mode, while others offer the possibility of storing the profile data and 
consequently can be operated in an "off-line" mode, often combined with 
the possibility of introducing slope corrections. Also the presentation 
of height information, derived during the orthophoto process, varies 
considerably for the different systems. Some instruments produce drop 
lines, which are used for subsequent manual construction of contour lines, 
while others provide a means to directly print sections of contour lines 
or to store the profile data in digital form for subsequent computational 
formation of terrain models or contour lines. Similarly, various tech- 
niques for image transfer are being used, such as direct optical projection, 
optical image transfer and electronic image transfer. 
This International Orthophoto Experiment was organized by Working 
Group II-4 of the International Society of Photogrammetry in order to 
collect information on the quality of products provided by commercially 
available orthophoto systems. Particular emphasis was put on the analysis 
of planimetric orthophoto accuracy and the quality of vertical terrain 
information obtained from various orthophoto production processes. 
In the experiment a photographic stereopair was used, taken 
over a test area specially established for this purpose in the vicinity 
of Ottawa, Canada. To ensure that the orthophotos used in the test could 
be regarded as representative of the particular instrument, the manufac- 
turers were asked to cooperate in this experiment by taking the respon- 
sibility for producing both the orthophotos and height data on their own 
equipment. The response by most manufacturers was very favorable and prompt 
throughout the project, for which the authors wish to express their 
appreciation and thanks. 
The actual evaluation work was conducted in the laboratories of 
the National Research Council of Canada. Mr. M.C. van Wijk supervised