Exposure interval (in seconds) for 60-percent
overlap
Ground gained
Ground coverage per inch of negative and
exposure intervals
When you intend to use the BM-38A computer,
refer to the Photographic Computer Instruction Book,
RC-025063, for detailed instructions.
SCALE
Usually, the area to be mapped is indicated on a
chart and maximum boundaries are provided. The scale
fraction of this chart, or its linear scale, provides
important information. The amount of area to be
covered can be determined from one of these scales.
The scale of a map is indicated as a common
fraction or as a ratio. For example, the scale may be
1/10,000 or 1:10,000 on the map. In either case, the
scale is read "one to ten thousand." This scale indicates
that one unit of measure on the map is equal to 10,000
of the same units on the ground.
One problem in aerial mapping is locating the scale
of the mosaic map. When the required scale is
provided, then the altitude and focal length must be
determined to get the required scale. The scale of a
photographic mosaic map is calculated as follows:
S = Scale of the map
F = Focal length of the lens
A = Altitude above the ground
With F (in inches), A (in feet) must be multiplied
by 12 to convert to the same unit of measurement
(inches).
S=
F
12A
Example: What is the scale of a map taken from an
altitude of 5,000 feet, using a 6-inch lens.
S=
6
=
=
6
1
12
5,000
60,000
10,000
Therefore, the scale is 1/10,000. That means 1 inch on
the photograph equals 10,000 inches on the ground.
FORWARD OVERLAP
To ensure complete coverage of the area, you
should take each photograph in each flight line or strip
so it overlaps both the preceding photograph and the
following photograph. The amount of overlap on each
photograph is approximately 60 percent. Creating this
overlap ensures that the strip contains no blank areas
(fig. 4-17).
The overlap also serves another important function.
In the construction of a mosaic map, only the central
area of each print is used. Only the central area is used
because the middle areas of all vertical photographs are
the area of truest reproduction of terrain. (See fig.
4-18.)
In figure 4-18, the aircraft is flying over a mountain
while making a series of vertical photographs. For all
practical purposes, when the aircraft is directly over the
mountain, a perfect reproduction of the mountain is
obtained. Pictures taken before and after the one
directly over the mountain show the near side of the
mountain clearly, but very little, if any, of the far side.
This is caused by the different camera positions in
respect to the subject.
Scale is affected by this difference of camera
positions. It is practically impossible to match the
edges of prints when these distortions of the terrain are
present. Therefore, the outer area (toward the edges of
the print) is discarded and the inner 40 percent of each
print is used. Another important reason for using only
the center area of the prints is that stereoscopic
measurement associated with either contour mapping or
photographic interpretation requires the highest degree
of accuracy.
Since a 60-percent overlap is created, only 40
percent of the ground-gained forward (GGF) is usable
in each negative. For example, a 5-
5-inch negative
has a usable image area of 2 inches. (5.0
0.40 = 2.)
To find the actual amount of usable GGF in each
negative, multiply the ground coverage by 0.40. For
example, using the IFGA formula, you have determined
that the ground coverage for each negative is 9,000 feet.
The usable GGF in each negative is 3,600 feet (9,000
0.40 = 3,600).
SIDE LAP
The area that you are photographing for a mosaic
map may be wide and cannot be photographed in one
strip. The aircraft must fly a number of side-by-side
strips to get complete coverage so none of the area is
4-20
