General Information - Cammenga PRECISION LENSATIC Instructional Booklet

3. Magnetic Declination - The horizontal angle (difference in
degrees) between Magnetic North, Grid North, and True
North. Magnetic declination varies from area to area and
from time to time: Generally about 1' (one minute) per year
(There are 60 minutes to one degree). The declination will be
shown in the marginal information on your map. When the
lensatic compass is used with a map, an adjustment should
be made to allow for the declination.
J. GENERAL INFORMATION
Readings should never be taken near visible masses of iron
or electrical circuits, because of their effects on the compass
magnet. The following are suggested as approximate safe
distances to insure proper functioning of the compass:
a. High tension power lines
b. Car, truck, camper
c. Telephone lines, wire fences
d. Rifle, metal boxes, etc.
The map scale index is usually found in the marginal
information on a map. Corresponding ground distances of
some commonly used map scales are shown below:
Fractional Scale
1:24,000
1:25,000
1:50,000
1:62,500
1:100,000
1:125,000
1:250,000
Simple Conversion
1 in = 2000 ft
1 mm = 100 m
1 mm = 50 m
1 in ~ 1 mi
1 cm = 1 km
1 in ~ 2 mi
1 in ~ 4 mi
55 meters (60 yards)
18 meters (20 yards)
10 meters (11 yards)
0.5 meters (1.5 feet)
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