of individual plates for departures from flatness, thereby generating appro- 
priate corrections for subsequent application in the data reduction. Indeed, 
this has been done routinely for years at GSI. To be valid, this procedure 
requires, of course, that the surface of the plate not be significantly 
altered from its normal state when pressed against the focal frame of the 
camera (a condition not necessarily satisfied by all commercial cameras). 
From the foregoing it is clear that as far as metric considerations are 
concerned, plates and film have mutually offsetting advantages and dis- 
advantages. With plates one has excellent dimensional stability, but inade- 
quate flatness, a deficiency which must be compensated for by means of 
special measurements of spot elevations on individual plates. With film, 
systematic departures from flatness are essentially the same from frame to 
frame, being determined almost wholly by the vacuum platen itself, and thus 
are amenable to correction through a one-time calibration of the platen. 
On the other hand, dimensional stability of film is inferior and must be 
counteracted by means of measurements of reseau images on individual frames. 
Thus, measurement of reseau images on film and measurement of spot elevations 
on plates essentially offset one another insofar as overall efficiency of 
data extraction is concerned. On the balance, then, film emerges as the 
more attractive medium for close-range photogrammetry. 
2.2 THE RESEAU PLATEN 
In its standard version the GSI reseau platen incorporates a 7 x 7 
array of reseau projectors. Upon actuation of the camera shutter the 
reseau projectors are illuminated by an electroluminescent panel and project 
the array of reseau images through the backside of the film. On the nega- 
tive each reseau mark consists of a black dot about 75ym in diameter 
surrounded by a circle about 500um in diameter (Figure 3). The projected: 
circle serves two functions. One is the obvious function of aiding in the 
location and identification of the reseau dots. The other is for detection 
of possible lack of contact of the film with the platen due to partial or 
total failure of vacuum flattening. Because the circle is centrally pro- 
jected onto the film, the diameter of the image of the circle is directly 
dependent on the distance between the reseau projector and the film. In 
fact, the angle subtended by the circle has been chosen so that a change of 
lum in the distance of the photographic emulsion from the platen will cause 
a change of lum in the diameter of the image of the reseau circle. Thus the 
reseau circles provide an extremely sensitive system for monitoring confor- 
mity of contact between film and platen. 
The reseau marks are spaced evenly at 58 mm intervals throughout the 
format. On option, denser distributions of reseau marks are available, 
notably a 169 (13 x 13) point array and an 85 point array (every other point 
of a 13 x 13 array). By virtue of a special calibration using standards 
traceable to the U.S. National Bureau of Standards, the x,y coordinates of 
the reseau images are provided to an rms accuracy of 0.50ym. These cali- 
brated values are exercised in an optimal fashion in STARS software to 
generate accurate corrections for film deformation. 
The platen is fabricated from the ultra-stable alloy Precident 71. Its 
surface is hard coated with Teflon-S and finished to such a degree of flat- 
ness that no point departs from a best fitting plane by more than 5um; the 
overall rms departure of the surface from a best fitting plane is under 
2.5um. To make it possible to account for residual departures from flatness, 
72