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ADVANCED NAVIGATION
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FM 3-25-26 Map Reading and
Land Navigation
NAVIGATION EQUIPMENT AND METHODS
Compasses are the primary navigation tools to use when moving in an
outdoor world where there is no other way to find directions. Soldiers should
be thoroughly familiar with the compass and its uses. Part One of this
manual discussed the techniques of map reading. To complement these
techniques, a mastery of field movement techniques is essential. This chapter
describes the lensatic compass and its uses, and some of the field expedient
methods used to find directions when compasses are not available.
9-1. TYPES OF COMPASSES
The lensatic compass is the most common and simplest instrument for measuring
direction.
It is discussed in detail in paragraph 9-2. The artillery M2 compass is a
special-purpose
instrument designed for accuracy; it will be discussed in Appendix G. The
wrist/pocket
compass is a small magnetic compass that can be attached to a wristwatch band.
It contains
a north-seeking arrow and a dial in degrees. A protractor can be used to
determine azimuths
when a compass is not available. However, it should be noted that when using the
protractor
on a map, only grid azimuths are obtained.
9-2. LENSATIC COMPASS
The lensatic compass (Figure 9-1) consists of three major parts: the cover, the
base, and the
lens.
Figure 9-1. Lensatic compass.
a. Cover. The compass cover protects the floating dial. It contains the sighting
wire
(front sight) and two luminous sighting slots or dots used for night navigation.
b. Base. The body of the compass contains the following movable parts:
(1) The floating dial is mounted on a pivot so it can rotate freely when the
compass is
held level. Printed on the dial in luminous figures are an arrow and the letters
E and W. The
arrow always points to magnetic north and the letters fall at east (E) 90° and
west (W) 270°
on the dial. There are two scales; the outer scale denotes mils and the inner
scale (normally
in red) denotes degrees.
(2) Encasing the floating dial is a glass containing a fixed black index line.
(3) The bezel ring is a ratchet device that clicks when turned. It contains 120
clicks when
rotated fully; each click is equal to 3°. A short luminous line that is used in
conjunction with
the north-seeking arrow during navigation is contained in the glass face of the
bezel ring.
(4) The thumb loop is attached to the base of the compass.
c. Lens. The lens is used to read the dial, and it contains the rear-sight slot
used in
conjunction with the front for sighting on objects. The rear sight also serves
as a lock and
clamps the dial when closed for its protection. The rear sight must be opened
more than 45°
to allow the dial to float freely.
NOTE: When opened, the straightedge on the left side of the compass has a
coordinate
scale; the scale is 1:50,000 in newer compasses.
WARNING: Some older compasses will have a 1:25,000 scale. This scale can
be used
with a 1:50,000-scale map, but the values read must be halved. Check the
scale.
9-3. COMPASS HANDLING
Compasses are delicate instruments and should be cared for accordingly.
a. Inspection. A detailed inspection is required when first obtaining and using
a
compass. One of the most important parts to check is the floating dial, which
contains the
magnetic needle. The user must also make sure the sighting wire is straight, the
glass and
crystal parts are not broken, the numbers on the dial are readable, and most
important, that
the dial does not stick.
b. Effects of Metal and Electricity. Metal objects and electrical sources can
affect the
performance of a compass. However, nonmagnetic metals and alloys do not affect
compass
readings. The following separation distances are suggested to ensure proper
functioning of
a compass:
High-tension power lines ........................................55 meters.
Field gun, truck, or tank ..........................................18 meters.
Telegraph or telephone wires and barbed wire .......10 meters.
Machine gun............................................................2 meters.
Steel helmet or rifle.................................................1/2 meter.
c. Accuracy. A compass in good working condition is very accurate. However, a
compass has to be checked periodically on a known line of direction, such as a
surveyed
azimuth using a declination station. Compasses with more than 3° + variation
should not be
used.
d. Protection. If traveling with the compass unfolded, make sure the rear sight
is fully
folded down onto the bezel ring. This will lock the floating dial and prevent
vibration, as
well as protect the crystal and rear sight from damage.
9-4. USING A COMPASS
Magnetic azimuths are determined with the use of magnetic instruments, such as
lensatic and
M2 compasses. The techniques employed when using the lensatic compass are as
follows:
a. Using the Centerhold Technique. First, open the compass to its fullest so
that the
cover forms a straightedge with the base. Move the lens (rear sight) to the
rearmost position,
allowing the dial to float freely. Next, place your thumb through the thumb
loop, form a
steady base with your third and fourth fingers, and extend your index finger
along the side
of the compass. Place the thumb of the other hand between the lens (rear sight)
and the bezel
ring; extend the index finger along the remaining side of the compass, and the
remaining
fingers around the fingers of the other hand. Pull your elbows firmly into your
sides; this will
place the compass between your chin and your belt. To measure an azimuth, simply
turn
your entire body toward the object, pointing the compass cover directly at the
object. Once
you are pointing at the object, look down and read the azimuth from beneath the
fixed black
index line (Figure 9-2). This preferred method offers the following advantages
over the
sighting technique:
(1) It is faster and easier to use.
(2) It can be used under all conditions of visibility.
(3) It can be used when navigating over any type of terrain.
(4) It can be used without putting down the rifle; however, the rifle must be
slung well
back over either shoulder.
(5) It can be used without removing eyeglasses.
Figure 9-2. Centerhold technique.
b. Using the Compass-to-Cheek Technique. Fold the cover of the compass
containing
the sighting wire to a vertical position; then fold the rear sight slightly
forward. Look
through the rear-sight slot and align the front-sight hairline with the desired
object in the
distance. Then glance down at the dial through the eye lens to read the azimuth
(Figure 9-3).
NOTE: The compass-to-cheek technique is used almost exclusively for
sighting, and it
is the best technique for this purpose.
Figure 9-3. Compass-to-cheek technique.
c. Presetting a Compass and Following an Azimuth. Although different models of
the lensatic compass vary somewhat in the details of their use, the principles
are the same.
(1) During daylight hours or with a light source:
(a) Hold the compass level in the palm of the hand.
(b) Rotate it until the desired azimuth falls under the fixed black index line
(for example,
320°), maintaining the azimuth as prescribed (Figure 9-4).
(c) Turn the bezel ring until the luminous line is aligned with the
north-seeking arrow.
Once the alignment is obtained, the compass is preset.
(d) To follow an azimuth, assume the centerhold technique and turn your body
until the
north-seeking arrow is aligned with the luminous line. Then proceed forward in
the direction
of the front cover's sighting wire, which is aligned with the fixed black index
line that
contains the desired azimuth.
Figure 9-4. Compass preset at 320 degrees.
(2) During limited visibility, an azimuth may be set on the compass by the click
method.
Remember that the bezel ring contains 3° intervals (clicks).
(a) Rotate the bezel ring until the luminous line is over the fixed black index
line.
(b) Find the desired azimuth and divide it by three. The result is the number of
clicks that
you have to rotate the bezel ring.
(c) Count the desired number of clicks. If the desired azimuth is smaller than
180°, the
number of clicks on the bezel ring should be counted in a counterclockwise
direction. For
example, the desired azimuth is 51°. Desired azimuth is 51°÷ 3 = 17 clicks
counterclockwise. If the desired azimuth is larger than 180°, subtract the
number of degrees
from 360° and divide by 3 to obtain the number of clicks. Count them in a
clockwise
direction. For example, the desired azimuth is 330°; 360°-330° = 30 ÷3 = 10
clicks
clockwise.
(d) With the compass preset as described above, assume a centerhold technique
and
rotate your body until the north-seeking arrow is aligned with the luminous line
on the bezel.
Then proceed forward in the direction of the front cover's luminous dots, which
are aligned
with the fixed black index line containing the azimuth.
(e) When the compass is to be used in darkness, an initial azimuth should be set
while
light is still available, if possible. With the initial azimuth as a base, any
other azimuth that
is a multiple of three can be established through the use of the clicking
feature of the bezel
ring.
NOTE: Sometimes the desired azimuth is not exactly divisible by three,
causing an option
of rounding up or rounding down. If the azimuth is rounded up, this causes an
increase in the value of the azimuth, and the object is to be found on the left.
If
the azimuth is rounded down, this causes a decrease in the value of the azimuth,
and the object is to be found on the right.
d. Bypassing an Obstacle. To bypass enemy positions or obstacles and still stay
oriented, detour around the obstacle by moving at right angles for specified
distances.
(1) For example, while moving on an azimuth of 90° change your azimuth to 180°
and
travel for 100 meters. Change your azimuth to 90°and travel for 150 meters.
Change your
azimuth to 360°and travel for 100 meters. Then, change your azimuth to 90°and
you are back
on your original azimuth line (Figure 9-5).
Figure 9-5. Bypassing an obstacle.
(2) Bypassing an unexpected obstacle at night is a fairly simple matter. To
make a 90°
turn to the right, hold the compass in the centerhold technique; turn until the
center of the
luminous letter E is under the luminous line (do not move the bezel ring). To
make a 90° turn
to the left, turn until the center of the luminous letter W is under the
luminous line. This does
not require changing the compass setting (bezel ring), and it ensures accurate
90° turns.
e. Offset. A deliberate offset is a planned magnetic deviation to the right or
left of an
azimuth to an objective. Use it when the objective is located along or in the
vicinity of a
linear feature such as a road or stream. Because of errors in the compass or in
map reading,
the linear feature may be reached without knowing whether the objective lies to
the right or
left. A deliberate offset by a known number of degrees in a known direction
compensates
for possible errors and ensures that upon reaching the linear feature, the user
knows whether
to go right or left to reach the objective. Ten degrees is an adequate offset
for most tactical
uses. Each degree offset moves the course about 18 meters to the right or left
for each
1,000 meters traveled. For example, in Figure 9-6, the number of degrees offset
is 10. If the
distance traveled to "x" in 1,000 meters, then "x" is located about 180 meters
to the right of
the objective.
Figure 9-6. Deliberate offset to the objective.
9-5. FIELD-EXPEDIENT METHODS
When a compass is not available, different techniques should be used to
determine the four
cardinal directions.
a. Shadow-Tip Method.
(1) This simple and accurate method of finding direction by the sun consists of
four basic
steps (Figure 9-7).
Figure 9-7. Determining directions and time by shadow.
Step 1. Place a stick or branch into the ground at a level spot where a
distinctive shadow
will be cast. Mark the shadow tip with a stone, twig, or other means. This first
shadow mark
is always the west direction.
Step 2. Wait 10 to 15 minutes until the shadow tip moves a few inches.
Mark the new
position of the shadow tip in the same way as the first.
Step 3. Draw a straight line through the two marks to obtain an
approximate east-west
line.
Step 4. Standing with the first mark (west) to your left, the other
directions are simple;
north is to the front, east is to the right, and south is behind you.
(2) A line drawn perpendicular to the east-west line at any point is the
approximate
north-south line. If you are uncertain which direction is east and which is
west, observe this
simple rule--the first shadow-tip mark is always in the west direction,
everywhere on earth.
(3) The shadow-tip method can also be used as a shadow clock to find the
approximate
time of day (Figure 9-7 on page 9-7).
(a) To find the time of day, move the stick to the intersection of the east-west
line and
the north-south line, and set it vertically in the ground. The west part of the
east-west line
indicates 0600 hours, and the east part is 1800 hours, anywhere on earth,
because the basic
rule always applies.
(b) The north-south line now becomes the noon line. The shadow of the stick is
an hour
hand in the shadow clock, and with it you can estimate the time using the noon
line and the
6 o'clock line as your guides. Depending on your location and the season, the
shadow may
move either clockwise or counterclockwise, but this does not alter your manner
of reading
the shadow clock.
(c) The shadow clock is not a timepiece in the ordinary sense. It makes every
day 12
unequal hours long, and always reads 0600 hours at sunrise and 1800 hours at
sunset. The
shadow clock time is closest to conventional clock time at midday, but the
spacing of the
other hours compared to conventional time varies somewhat with the locality and
the date.
However, it does provide a satisfactory means of telling time in the absence of
properly set
watches.
(d) The shadow-tip system is not intended for use in polar regions, which the
Department
of Defense defines as being above 60° latitude in either hemisphere. Distressed
persons in
these areas are advised to stay in one place so that search/rescue teams may
easily find them.
The presence and location of all aircraft and ground parties in polar regions
are reported to
and checked regularly by governmental or other agencies, and any need for help
becomes
quickly known.
b. Watch Method.
(1) A watch can be used to determine the approximate true north and true south.
In the
north temperate zone only, the hour hand is pointed toward the sun. A south line
can be
found midway between the hour hand and 1200 hours, standard time. If on daylight
saving
time, the north-south line is found between the hour hand and 1300 hours. If
there is any
doubt as to which end of the line is north, remember that the sun is in the east
before noon
and in the west after noon.
(2) The watch may also be used to determine direction in the south temperate
zone;
however, the method is different. The 1200-hour dial is pointed toward the sun,
and halfway
between 1200 hours and the hour hand will be a north line. If on daylight saving
time, the
north line lies midway between the hour hand and 1300 hours (Figure 9-8).
Figure 9-8. Determining direction by using
a watch.
(3) The watch method can be in error, especially in the lower latitudes, and may
cause
circling. To avoid this, make a shadow clock and set your watch to the time
indicated. After
traveling for an hour, take another shadow-clock reading. Reset your watch if
necessary.
c. Star Method.
(1) Less than 60 of approximately 5,000 stars visible to the eye are used by
navigators.
The stars seen as we look up at the sky at night are not evenly scattered across
the whole sky.
Instead they are in groups called constellations.
(2) The constellations that we see depends partly on where we are located on the
earth,
the time of the year, and the time of the night. The night changes with the
seasons because
of the journey of the earth around the sun, and it also changes from hour to
hour because the
turning of the earth makes some constellations seem to travel in a circle. But
there is one star
that is in almost exactly the same place in the sky all night long every night.
It is the North
Star, also known as the Polar Star or Polaris.
(3) The North Star is less than 1° off true north and does not move from its
place because
the axis of the earth is pointed toward it. The North Star is in the group of
stars called the
Little Dipper. It is the last star in the handle of the dipper. There are two
stars in the Big
Dipper, which are a big help when trying to find the North Star. They are called
the Pointers,
and an imaginary line drawn through them five times their distance points to the
North Star.
There are many stars brighter than the North Star, but none is more important
because of its
location. However, the North Star can only be seen in the northern hemisphere so
it cannot
serve as a guide south of the equator. The farther one goes north, the higher
the North Star
is in the sky, and above latitude 70°, it is too high in the sky to be useful
(Figure 9-9).
Figure 9-9. Determining direction by the
North Star and Southern Cross.
(4) Depending on the star selected for navigation, azimuth checks are necessary.
A star
near the north horizon serves for about half an hour. When moving south, azimuth
checks
should be made every 15 minutes. When traveling east or west, the difficulty of
staying on
azimuth is caused more by the likelihood of the star climbing too high in the
sky or losing
itself behind the western horizon than it is by the star changing direction
angle. When this
happens, it is necessary to change to another guide star. The Southern Cross is
the main
constellation used as a guide south of the equator, and the above general
directions for using
north and south stars are reversed. When navigating using the stars as guides,
the user must
know the different constellation shapes and their locations throughout the world
(Figure 9-10
and Figure 9-11 on page 9-12).
Figure 9-10. Constellations, northern hemisphere.
Figure 9-11. Constellations, southern hemisphere.
9-6. GLOBAL POSITIONING SYSTEM
The GPS is a space-based, global, all-weather, continuously available, radio
positioning
navigation system. It is highly accurate in determining position location
derived from signal
triangulation from a satellite constellation system. It is capable of
determining latitude,
longitude, and altitude of the individual user. It is being fielded in
hand-held, manpack,
vehicular, aircraft, and watercraft configurations. The GPS receives and
processes data from
satellites on either a simultaneous or sequential basis. It measures the
velocity and range
with respect to each satellite, processes the data in terms of an
earth-centered, earth-fixed
coordinate system, and displays the information to the user in geographic or
military grid
coordinates.
a. The GPS can provide precise steering information, as well as position
location. The
receiver can accept many checkpoints entered in any coordinate system by the
user and
convert them to the desired coordinate system. The user then calls up the
desired checkpoint
and the receiver will display direction and distance to the checkpoint. The GPS
does not
have inherent drift, an improvement over the Inertial Navigation System, and the
receiver
will automatically update its position. The receiver can also compute time to
the next
checkpoint.
b. Specific uses for the GPS are position location; navigation; weapon location;
target
and sensor location; coordination of firepower; scout and screening operations;
combat
resupply; location of obstacles, barriers, and gaps; and communication support.
The GPS
also has the potential to allow units to train their soldiers and provide the
following:
• Performance feedback.
• Knowledge of routes taken by the soldier.
• Knowledge of errors committed by the soldier.
• Comparison of planned versus executed routes.
• Safety and control of lost and injured soldiers.
(See Appendix J for more information on the GPS.)
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