data on the image quality of the different orthophoto systems. Such a 
study would have required additional tests and uniform photographic pro- 
cessing, a rather cumbersome requirement in this kind of an international 
project. 
Nevertheless an attempt has been made to obtain some informa- 
tion on differences in image quality as discernible in a visual inspection 
of the submitted orthophotos. 
The following factors of orthophoto image quality were con- 
sidered: 
- Image definition of suitable image detail. The reseau crosses, 
which were among the finest lines that could be detected in the 
original photographs, were used for this purpose. The line thick- 
ness of crosses was approximately 25 um in the original 1:10 000 
photographs. 
Double exposures and discontinuities of linear details due to 
excessive slit length. 
Image blur due to image movement, caused primarily by excessive 
slit width. 
a. Photographic Quality of Image Detail 
The reseau crosses provideda means to assess the uniformity of 
the image quality over the entire orthophoto. Reseau crosses,  imaged 
against an even grey background (farm fields) were selected in six loca- 
tions, near the four corners of the orthophoto and near the principal 
points of both photographs. The line width of the reseau crosses in the 
orthophoto, which included their blurred edges, was measured using a 40x 
enlargement microscope equipped with a scale. This width was measured 
for both lines of each reseau cross and the average values was used in 
the analysis. The evaluation was based on the originally submitted 
orthophoto negatives and the 1:2 500 scale positive transparencies, 
obtained either by contact printing or enlargement on the Borrowdale 
Cartographic Camera. The following results were obtained: 
- The orthophotos produced by optical image transfer systems 
generally indicated a better image quality than those produced by 
electronic image transfer or by direct optical projection. Con- 
verted to the original scale 1:10 000 the line width of the reseau 
marks depicted in orthophotos produced on the Wild OR 1, Wild PPO-8 
and Zeiss Jena Orthophot was approximately 33 um as compared with 
the line width of 25 um in the original diapositives. The ortho- 
photos produced on the Gestalt Photo Mappers I and II (electronic 
image transfer) and the .Zeiss Ortho-3 (optical projection)