Advanced Photography Course

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.

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

contents

Advanced Photography Course

<- previous page

next page ->


DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> 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 contents Advanced Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
	WWW.DOFMASTER.COM © 2006 Don Fleming. All rights reserved.