10 
c. Illumunation at nominal projection distance pro 
duced by a 100 watt light source at 12 volts 
and measured normal to the incident light, in 
footcandles. 
TABLE III 
0° 20° 30° 40° 50° 55 
36 32 30 20 23 8 
These values are averages of measurements dis 
tributed over the entire area of projection 
and made of a series of projection systems. 
d. Model flatness. The use of grid plates and 
the measurement of vertical deformations of a 
grid model is usually considered the supreme 
performance test of a plotting instrument. 
This is not valid without certain reservations 
in the case of super wide angle projections. 
By virtue of the improved projection geometry 
based on a base-height ratio of 1:1 or even 
better and of the superior quality of the pro 
jected grid image the coincidence setting of 
longitudinal and transverse grid lines is more 
critical than experienced with 90° projection 
geometry. Straightness,line spacing and mutual 
perpendicularity of grid lines must meet more 
demanding specifications than ever before. 
Above all, the lines cannot be ruled into the 
glass base. They must have no depth. Grid 
model tests were, therefore, not conclusive, 
until grids of higher quality became available. 
Terrain models of aerial photography over test 
areas of abundant ground control were, therefore 
preferred. They confirmed, that model defor 
mations due to residual distortion values shown