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