indicated a reseaü line width of 45 um. For orthophotos produced 
on the Zeiss GZ-1 (optical projection) and the Galileo Orthophoto 
Simplex (optical image transfer) this figure was 50 um while for 
the Kelsh K320 Orthoscan and the Ortho SFOM 9300 (optical pro- 
jection) a line width of 60 um was measured. 
No significant difference was detected in the image quality of 
the original orthophoto negatives and the diapositive copies, 
obtained by contact printing or enlargement in the cartographic 
camera. 
The orthophotos produced by optical or electronic image transfer 
indicated a uniform image quality over the entire orthophoto. This 
was not the case for the orthophotos produced by direct optical pro- 
jection. For the orthophotos produced on the K320 Orthoscan, the 
Ortho SFOM 9300 and the Zeiss GZ-1 the image quality near the 
principal point was approximately 20 - 25% better than in the 
corners. For the orthophotos produced on the Zeiss Ortho-3 this 
difference was approximately 10%. 
b. Double Exposure and Image Discontinuities 
These image defects, which affect both the definition and geomet- 
rical quality of the orthophoto image, depend on the length of the scanning 
slit, on the slope angles, andon the fact whether or not slope corrections 
normal to the profile directions are included in the orthophoto process. 
In the Ripon test area where maximum slopes are approximately 30°, discon- 
tinuities in linear details and distances between double exposed images 
amount to approximately 252 of the slit length for instruments in which 
no slope correction was used. 
No double exposures and discontinuities were detected in the 
orthophotos produced on the Gestalt Photo Mappers I and II except a few 
distortions in features, located above the terrain level (building roofs). 
Double exposures and image discontinuities in orthophotos pro- 
duced on the Wild OR 1 and the Zeiss GZ-1, in which slope correction is’ 
applied, were very small and did not exceed 0.1 mmand 0.3 mm, respectively, 
(in the orthophoto scale 1:2 500). 
c. Orthophoto Image Blur 
Blur in the orthophoto image is caused by image movement during the 
photographic exposure and depends on the slit dimension parallel to the 
scanning direction (slit width). It has been found that, for the steeper 
parts of the Ripon Test Area, the orthophoto image quality was seriously 
  
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