converting airspeed (in knots) to feet is 1.7 (kts
ft/sec). For example, when the airspeed is 125 knots,
the ground speed, in feet, is 212.5 feet per second (125
1.7 = 212.5).
Head winds and tail winds must also be considered.
When head winds are present, you should subtract the
head wind from the airspeed. Tail winds should be
added to the airspeed. For example, when the aircraft
is flying north at 125 knots and the wind is blowing from
the north at 10 mph, then the corrected airspeed is 115
knots (125 - 10= 115). When the air craft is flying south,
then the corrected airspeed is 135 knots (125 + 10 =
135). The corrected airspeed must be used to find the
ground speed, in feet.
For the time interval between exposures, the
following formula should be used:
T = Time in seconds
D = Ground-gained forward
S = Ground speed in feet per second
NOTE: When the interval between exposures can
be accomplished in full seconds only, the tenths of a
second should be dropped. By doing so, slightly more
than the required 60-percent forward overlap is
provided. This can be advantageous.
SCALE OF THE FINISHED
The scale of the finished mosaic map is determined
by using the following formula:
F = Lens focal length
A = Altitude
The answer derived by using this formula gives a
representative fraction (RF) in like-units. Notice that
you must convert the altitude to inches, since the lens
focal length is normally in inches. For example, you
used a 10-inch focal-length lens for a mapping mission
flown at 5,000 feet. You can determine the scale of the
finished mosaic as follows:
The scale of the finished mosaic is 1/6,000 (1:6,000).
To reinforce the mission planning procedures, you
can use the following example:
You are assigned to assist in the mission
calculations required for a recon mapping mission. You
are briefed on the mission and the following
information is provided:
Area to be mapped is 10 nautical miles east and west
by 20 nautical miles north and south.
Forward overlap required is 60 percent.
Side lap required is 40 percent.
Lens focal length is 12 inches.
Negative size is 9
True airspeed of aircraft is 140 knots.
The wind is from the north at 15 knots.
The scale of the chart used to plan and fly the
mission is 1/50,000 (1:50,000).
The required scale is 1/12,000 (1: 12,000).
A graphic scale representing 3,000 feet is required
on the printed mosaic map.
1. Determine the altitude. The first step in this
problem is to determine the altitude at which the aircraft
must fly to obtain the required scale of 1/12,000. The
IFGA formula used to determine the altitude is as
I = G orA=FG
Since G (ground coverage) is not known, you must
substitute the required scale (1/12,000) for it. At a
required scale of 1/12,000, each unit of I (on the film
plane) records 12,000 units of G. Since A is measured
in feet, you must divide your answer by 12 to get the
units in feet.
12,000 = 144,000 =144,000
Divide by 12 to get altitude units in feet
Advanced Photography Course