the maximum flight height above the lowest elevation by an amount equal to
60 percent of the maximum relief height. Under such conditions and in the case
considered, the optimum flight height at which the photographs would be taken
is 1,500 feet (457 meters) above a plane passing through an elevation 60 per-
cent of the relief height higher than the lowest elevation in the stereoscopic
area. With the instrument being considered, this optimum flight height will
permit mapping at a manuscript scale of 50 feet to one inch (1:600). A flight
height of 2,660 feet (811 meters) above the lowest elevation would provide an
optimum flight height of 2,400 feet (732 meters) and permit mapping at a manu-
script scale of 80 feet to one inch (1:960), or in the metric System mapping
at a scale of 1:1,000. Consequently, wherever the actual relief height to
flight height ratio in the area of photography and mapping is less than listed
in table 5, this ratio causes no difficulty in use of any of the instruments.
It is only in large scale mapping for engineering purposes and wherever the
relief demands it that this ratio must be considered and the scale of mapping
governed thereby.
Table 5. — Relief height to flight height ratios of
some double projection photogrammetric instruments
Photography
Projection Focal Length h/H
Instrument Ratio* (in.) Ratio
Bausch & Lomb Multiplex 2.4:1 6 0.36
Bausch & Lomb Balplex (525) 3.4: 6 0.28
Kelsh Stereoplotter 4:1 8.25 0.25
Kelsh Stereoplotter 5:1 8.25 0.21
Kelsh Stereoplotter 5:1 6 0.25
Bausch & Lomb Balplex (760) 5:1 6 0.25
Nistri Photocartograph 5:1 6 0.25
Kelsh Stereoplotter 7:1 6 0.22
Nistri Photocartograph 7:1 6 0.22
*The projection ratio is the number of times the stereoscopic model scale is
larger, at the optimum projection distance of the instrument, than the scale
of the vertical photography.
Accuracy and completeness tests of maps
Whenever maps are used without knowledge of their quality and accuracy,
there exists the possibility that their ultimate actual costs will be much more
than their initial costs of photography, ground control surveys, and compi-
lation. If the maps prove to be deficient in completeness and accuracy, there
will inevitably be added costs directly attributable to frustrations occurring
from their use; coping with their errors and inadequacies as they are discov-
ered during use; and having to perform, at a later time, such additional work,
as necessary, to adjust for the effects thereof. Whenever these effects are
not discovered and corrected before the design, and are actually not known
until after the location survey has been made and highway construction is to
be started or is underway, the added costs include costs resulting from work
required to make needed revisions in the design and to prepare and issue change
orders and extra work orders, costs of delays in construction while the pre-
ceding work is being accomplished, and the costs of all increases in construc-
tion quantities caused by the maps. Consequently, all maps should be tested to
ascertain their qualities. Rejection of maps, however, can be made only on the
basis of noncompliance with stipulations prestated in the form of appropriate
specifications before the mapping is undertaken.