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272
NOTES ON MILITARY MAP READING.
By
MAJOR A. P. BLENKINSOP.
Royal Army Medical Corps.
(Continued fT01n p. 49., vol. xiv.)
TRUE AND MAGNETIC NORTH.
IT is perhaps hardly necessary to state that the needle of the
compass does not point to the "true" but to the "magnetic"
north. The meridian or true north line never changes its direction;
but the magnetic north varies with the variations of the compass.
The deviation of the magnetic north line from the true north line
is known as the magnetic variation: The amount of this variation
is expressed by the degree of the angle formed by the intersection
of these two lines at the centre of the compass, and is said to be
either east or west according as the compass needle points either
to the east or west of true north. On all' military maps and
sketches the true and magnetic north are shown as in the accompanying conventional SIgn (fig. 9), which indicates a magnetic
variation of 15° W.
, FIG. 9.
" The variation is subject to two principal changes, annual and
posi tional."
" Annual Ghange.-In London the variation was 11° E. in 1576.
It then gradually decreased until in 1660 the magnetic needle pointed
to true north. The needle continued to move westward until in
1814 it reached its greatest westerly deviation (24° 30' W.). Since
1814 the needle has been moving eastward, and the variation has
been decreasing at the rate of some 7 {annually." In 1906 it was
about 15° W. in London and 18° W. in the west and north of
England.
"Positional Ghange.-The variation also changes according to
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273
A. P. Blenlcillsop
the position on the earth's surface. In northern India the variation
is about 2° E., while in the extreme south the needle points nearly
due north. In South Africa there is a variation of about 25° \V.,
and in France and Germany from 10° to 15° W."
In addition to these changes, the compass may be affected by
local magnetic attraction, due to the deflection of the needle by
masses of magnetic iron ore or of iron. In parts of South Africa,
and in some other countries where there are large deposits of iron
ore, the attraction is so strong that a compass is useless. In
England the error from this cause would seldom be more than 2°.
270
90
"0
"
'<?o
..?o
,/ "\iI,s~o
o
0)
180 ~
0
FIG. 10.
However, one should be careful not to make compass observations
in too close proximity to an iron gate, railway line, &c., or the
results may be misleading.
Different compasses may vary in their readings as much as two
or three degrees. This is due either to the dial not being accurately
fitted over the needle, or to the dial not being properly centred on
the pivot.
It is hardly necessary in these notes to describe the prismatic
compass as Royal Army Medical Corps officers usually do not carry
this instrument on manceuvres or field service, nor are. they called
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274
Notes on Military Map Reading
upon to prepare field sketches. However, fig. 10 is introduced to
show the various points of the compass and the corresponding
magnetic bearings in degrees.
It will be observed that there are thirty-two points corresponding
to the 360 degrees of the circle. Each point is therefore U!O from
the next one. Having committed the sequence of these points to
memory, and bearing in mind that the degrees are numbered from
left to right, like the figures of a watch, commencing at the north
point, one may readily convert a true direction to a magnetic
bearing.
For instance, if an order is given to advance west-north-west
(true), and it is known that the local magnetic variation is 16° W.,
west-north-west by the compass is two points north of west (270°).
270° + UtO + Il!O = 292to. Add 16° to allow for the westerly
variation, and the result, 308tO, will give the magnetic bearing of
the line of advance; that is to say, a direction of approximately a
point and a half north of west-north-west as shown by the compass.
Bearings.-True bearing is the angle a line makes with the true
north lille.
FIG. 11.
Magnetic bearing is the angle a line makes with the magnetic
north line.
In each case the angle is measured from north by east and south,
i.e., in the same direction as the hands of a watch move.
To Convert True to Magnetic Bearings and vice versd.-This
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A. P. Blenkinsop
275
may be done by adding or subtracting the variation as may be
required. Fig. 11 illustrates graphically the necessary calculation,
and explains how the student may confirm his result.
The true bearing of an object is 193°; what is the magnetic
bearing 1 Draw both north points and mark them as in fig. 11,
inserting the value of the variation, which is here taken as 16!-0 W.
Place the pencil on the true north line, and working east and using
o as the centre, draw an arc round until 193° is described at W.
Mark this arc 193°, and from the centre 0 draw a line to W.
Prolong 0 W to X. Then from the centre 0 describe an arc,
working east from the magnetic north line to X. It is obvious
that the arc drawn from the magnetic north line will be 161°
greater than that drawn from the true north line. But the arc
from the true north line = 193°. Therefore that from the magnetic
north line is 193° + 16!0 or 209!0; which is the magnetic bearing
acquired. The following are the rules for the conversion of
bearings:A.-When the Variation is West.
(1) To find true when given magnetic bearing. Subtract the
variation; if the result is minus, subtract it from 360°.
Examples. Magnetic bearing is 9°, find true.
Given variation 16!0 W.
9° - 1610
n°
360° - 7!0
3521° = true bearing.
(2) To find magnetic when given true bearing. Add the variation; if the result is greater than 360°, subtract that from it.
Examples. True bearipg is 349°, find magnetic.
349° + 16io = 365io
365!0 -- 360° = 5io = magnetic bearing.
B.-When the Variation is East.
(2) To find true bearing when given magnetic bearing. Add the
variation; if the result is greater than 360°, subtract 360° from it.
(2) To find the magnetic when given true bearing. Subtract
the variation; if the result is minus, subtract it from 360°.
==
To FIND THE DIRECTION OF TRUE NORTH.
This may be done without the aid of a compass :(a) By day, by an observation of the sun.
Cb) By night, by an observation of the stars.
By an Observation of the Sun.-" The sun, speaking generally,
rises in the east and sets in the west. Outside the Tropics, it is
at noon approximately due south of an observer in the northern'
hemisphere and approximately due north of an observer in the
19
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276
Notoes on Military Map Reading
southern hemisphere; the statement, however, regarding the rising
and setting of the sun is only accurate at the equinoxes; at other
times there will be less or more variation according to the altitude
and time of year, amounting in London to 38io north at midsummer,
and 38io south at midwinter."
The following is an accurate method of finding the true
north:" Lean a pole pointing northwards on two cross sticks (fig. 12).
From its tip drop a plummet hne to the ground at A."
FIG. 12.
"From. the point thus found as a centre and. at a convenient
radius describe a circle. Before noon watch the shadow of the pole
as it gradually gets shorter and shorter, till at last the top of the
shadow will just touch the circumference of the circle. Mark.this
spot with a picket. The shadow will continue to shorten until
12 o'clock, and will then lengthen again. Watch it as it creeps out
until it again just touches the circumference of the circle; mark
this spot with another picket. Between these two observations
the sun must have "culminated" or reached its highest point in
the heavens. If, therefore, the arc between the two pickets be
bis~cted and a line A B drawn from the centre of the circle to the
point of bisection, the direction of the true meridian or north and
south line, will be obtained."
"Then from some point C on the line B A carefully produced,
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A. P. Blenlcinsop
277
take an'observation of the two points A and B, and note the reading
of the compass; the difference between the compass reading and
360° (or 0°) will be the variation of the compass; if the compass
reads 100 it is evident that the compass bearing 360°, or magnetic
north, lies 10° W. of the true north, and that its variation is 10° W.;
if it reads 345° it is evident that the compass needle' (or bearing
360°) points 15° E. of true north. The ground on which this operation is carried out must be perfectly smooth and level." (" Notes
on Map Reading for Use in Army Schools.")
To find the Approximate True North with a Watch:"In the Northern Hemisphere.-Hold the watch horizontally
with the face upward. Point the hour hand at the sun. Then a
line from the centre of the dial to a point half-way between the
figure XII. and the pointer of the hour hand is approximately a
south line."
"In the Southern Hemisphere.-Hold the watch as before, but
in this case point the line from the centre of the dial to figure XII.
at the sun, then the line found as above is in this case approximately
a north line."
"Note.-This method is a very rough one. It should never be
used in the Tropics, and the higher the latitude, i.e., the further from
the Equator, the more reliable it is."
To roughly estimate the compass degrees from a watch, the
figure XII. may be taken as 360° (or 0°) and the interval between
each minute marked on tbe dial will correspond to an interval of 6°
as shown on the compass.
To find the Direction of True North by Night:" (a) In the No;thern Hemisphel'e.-Tbe Pole Star and all tbe
stars of the universe circle round an invisible point, P (fig. 13), the
Pole Star, at an angular distance there from of 1° 19'; it is evident,
therefore, that twice in every twenty-four hours the North Star, N.,
must come in the same vertical plane with it. Therefore by taking
the bearings of the North Star when it is vertically above, or below
the Pole, we are really taking the bearing of the Pole itself, i.e., of
true north. The North Star is in the same vertical plane with the
Pole when the star Zeta, ' ,,' the last but one from the end of the
tail of the' Great Bear,' is vertically above or below it. This can
be ascertained by a plumb line, and the direction being picketed out
on the ground, the bearing can be taken by daylight, and the variation of the compass ascertained."
" In order to identify tbe North Star, note the following diagram
(fig. 13) of the seven stars of the constellation known as the' Great
Bear.' "
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278
Notes on Milita171 Map Reading
N
*
p.I', ,
,,
,,
,,
,,
,,
,,
,,
,,
I
I
I
I
I
I
I
,
I
s*
*
*
*
I
*
FIG. 13.
a and b are the pointers, so-called because they point towards
the Pole or North Star, N., which is the last star in the tail of the
"Little Bear." S is the star Zeta, "
in the tail of the "Great
Bear."
(b) In the Southern Hemisphere.-Consider the Southern Cross
as a kite; prolong the greater axis four and a half times in the
direction of the tail, and the point reached will be within 10 of the
South Pole. If a piece of paper be marked off with nine equal
divisions on the edge, and held so that the first and third divisions
coincide with the head and tail stars respectively, the ninth division
will give the approximate South point.
s,"
THE PROTRACTOR.
Every officer who takes up the study of military map reading
should purchase a protractor and make himself thoroughly conversant with its various uses. This he can do by a trivial expenditure of money and time, for which he will be amply repaid. The
following description of the instrument has been taken from official
books:-'
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A. P. Blenlcinsop
279
The Service protractor is an instrument graduated on one side
with a series of degrees, similar to those of a compass, radiating
from the centre of the inner edge, which is marked with an arrow
head. It is made of boxwood and is exactly 6 inches in length.
The degrees up to 180 are shown by the outer row of figures, those
from 180 to 360 by the inner row.
To Lay-off a Bearing.-Place the protractor on the map with
the radiating centre, which is marked by an arrow-head, on the spot
from which the bearing is to be ascertained, the inner edge of the
instrument pointing north and south. For bearings up to 180°,
the graduated edge of the protractor is placed to the right or east;
for bearings 180° to 360°, the graduated edge of the protractor is
laid to the left or west.
To Take off a Distance.-There is also a scale of inches on
the Service protractor from which lengths measuring decimals of
inches, such as are required in constructing scales, can be measured.
The first place of decimals is given by the subdivisions on the bottom
line of the decimal scale, the second place by counting upwards the
required number along the diagonal line starting from the first
place of decimals. Thus, for a line 2'5 inches, put one point of the
dividers at the figure 2 on the main scale, and the other at the fifth
division on the bottom line of the decimal scale. For 2·55 count up
to the fifth horizontal lines on the fifth division in the decimal scale
and measure from that point along that horizontal line to the second
.
division on the main scale.
The protractor shows two scales of yards :(a) 2 inches = 1 mile, which can also be used by doubling distances for maps on the 4-inch scale; or by halving distances for the
1-inch scale.
Cb) 3 inches = 1 mile, which can also be used by doubling distances for the 6-inch scale. On both scales the primary divisions
show hundreds of yards.
Two other scales are provided for use with maps of lochroo and
~ooo' or for such factors as 5001000'
Information is given which would readily enable one to convert
foreign measures of length to the British standard, and a scale is
presented showing the horizontal. equivalents of various degrees of
slope with a vertical interval of one foot.
SETTING A MAP.
Before one can compare a map with the ground which it'
represents it is obvious that the map must be so arranged that
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280
Notes on Military Map Reading
the different railway lines, roads, &c., on it coincide in direction
with the corresponding objects on the ground; or, in other words,
the map must be set. A map is said to be set when it is laid out
to correspond with the ground, so that the true north on the map
points to the North Pole; or the magnetic north on the map
points in the same direction as (i.e., parallel to) the needle of the
compass. If the direction between features on the ground be now
compared with these directions as shown on the map, they will
be seen to be parallel. To express this somewhat differently, by
setting a map is meant placing it in such a position relating to
the ground it represents, that lines or "rays," drawn from the
point on the map denoting the position of the observer to any
other known points on the map, coincide in direction with the
imaginary lines proceeding from the eye of the observer to those
points as they are actually seen on the ground.
The following diagram (fig. 14) will more fully illustrate the
above definitions. It is introduced because it is absolutely
necessary that the student of map reading should have an
accurate knowledge of the subject under discussion, and that he
should fully understand both the theory and practice of map
setting.
o
N 360orO
,
270° W
>
,,
,,
0
,,
0
I------'-:'~-----__l£
90
,'P...
/,,/
,
,,
"'"
,
'" ,
,
",
S 180
o
FIG. 14.
Fig. 14 shows the four cardinal and intermediate points of the
compass. Supposing an observer, standing at P and facing
north, has a map spread out before him with the north of the map
pointing to the North Pole, it is obvious that the other points of
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A. P. Blenlcinsop
281
the compass on the map will correspond with the other points of
the compass on the ground, and that features seen on the ground
from P will coincide in direction with those features as represented on the map. Moreover, it is obvious that so long as the
map is undisturbed it will be found to be set to the country from
any point of view at which it may be looked at by the observer.
METHODS OF SETTING A MAP.
A. With a Oompass.-If the magnetic north line is shown on
the map, lay the compass over it, producing the line if necessary.
Then, without disturbing the compass, turn the map slowly round
until the north and south points of the magnetic line are exactly
under, or are exactly parallel to, the north and south points as
indicated by the needle of the compass. If the true north line only
is shown and you know the local variation of the compass, plot the
magnetic north on the map with a protractor and proceed as before.
If you have no protractor, lay the compass on the true north line
and turn the map until this line makes with the needle an angle
equal to the variation and on the correct side of it. Thus with a
variation of 17° W., the map would be turned until the true north
line was 17° to the right (or east) of the needle of the compass.
The sides of rectangular Ordnance maps of Great Britain are
drawn parallel to a true north and south line running through
Delamere Forest; in the extreme east and west counties they will
vary about 4° from a true north and south line; they are therefore
only approximately north and south. They may, however, in spite
of this possible error, be used as true north and south lines in
setting a map by this method.
B. Without a Compass.-Assuming the observer has ascertained
the point on the map which represents the position he occupies,
he may set the map by following these directions :(a) Identify on the map some distant conspicuous object which
can be seen in the surrounding landscape. This object (church
spire, windmill, &c.) should be as distant as possible.
(b) Draw a line on the map from the point representing your
position to the representation of the distant object chosen.
(c) Turn the map round the point, marking your position till
this line points to the distant object (fig. 15).
(Instead of actually drawing a line as directed in (b), a ruler
or straightedge may be adjusted so that its edge passes through the
two points (d) and (e). Th~ map is then turned until this edge
points directly from the eye of the observer to the distant object.)
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282
Notes on Military Map Reading
.... £
\'.\
,
. \"\",
,,
,,
FIG. 15.-d, Observer; e, distant object on map; E, distant object on ground.
Frequently the map can be easily set by placing the representation of a straight road or railway line on the map immediately over,
or in continuation of, the corresponding road or railway line on the
ground. Objects on either side of the road or line should then
be observed to see if they correspond in position with their representations on the map.
The map may be roughly set by adjusting it so as to correspond
with the north and south line as ascertained with a watch, and of
course this may be precisely accomplished if the true north point
is fixed by an observation of the sun or stars by the accurate
methods previously described (see figs. 12 and 1:3).
The following is a description of how to set a map, or check
the setting of a map, by what may be called the "sundial"
method. The sun at noon is due south in the Northern Hemisphere. Its angular speed from east to west, if measured on the
horizontal plane of the ground, may be roughly estimated at 15°
an hour. Hence at 3 p.m. the sun is approximately true southwest, for if the points of the compass are referred to (fig. 10), it will
be seen that south-west is 45° from south. If, therefore, on the
margin of the map a small circle is drawn to represent the compass,
with its north and south lines parallel to the border of the map,
and the principal points marked thereon, the shadow of a pin,
placed upright at south-west, should pass th:r:ough north-east at
3 o'clock, and if it does not do so, or nearly so, the map is not
set with the sun, and must be shifted until it so corresponds.
Extreme accuracy is only obtainable at noon. At that hour the
sun in this part of the world is exactly due south, and the shadow
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283
A. P. Blenlcinsop
of a pin, inserted vertically at south on the circle, may be relied
upon to point due north. For an hour before and an hour after
mid-day the calculations of 15° angular speed may also be fairly
relied upon j but at a greater interval of time the observation is
only approximately accurate. For military purposes it is, however,
sufficiently useful. (" Studies in Map Reading and Field Sketching," by Lieutenant-Colonel Wilkinson Shaw.)
.
To :FIND ONE'S POSITION ON THE MAP.
It may be remembered that when dealing with the way to set
a map by objects without a compass, it was assumed that the
observer knew his own position on the map. It may, however,
be necessary for him to fix his position, and to do so without
the aid of a compass or other ready means of setting the map.
This may be accomplished by what is known as the adjustment
or tracing paper method now to be described.
By Adjustment.-Three objects on the ground, which are shown
on the map, are selected (fig. 16, A BC).
,
,?C
,
,,
OB
,
\
,,
,
,,
\
\
,,
\
,,
" ,,
,,
,,
,,
\b
~
Tracing
paper
FIG. 16.
:c
x
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284
Notes on Military Map Reading
A piece of tracing or other transparent paper is spread out on
a flat surface in front of the observer, and a pin is driven into
this paper so as to stand vertically at any convenient point, x.
Then, from x, rays are drawn along the straight edge of a ruler,
which is carefully aligned by the observer's eye on each of these
selected objects in turn. The rays will appear as xa, xb, and xc in
the diagram. The tracing paper is now applied to the map and
is moved about until the rays xa, xb, and xc pass through the
representations of A, B, ana C. The point x is then pricked
through and marks the position of the observer on the map.
If the map can be set by the compass, the position of the
observer can be ,ascertained by "resection."
By Resection.-The map is first set. Two objects which are
observed on the ground (at A and B, fig. 17), and which are
0B
,
0A
\
I
I
I
\
I
\
,
\
I
I
\
\
I
\
\
,
\
I
I
,
\
I
I
,
I
I
I
\
I
\
I
x
FIG. 17.
marked on the map (at a and b), are selected. The straight
edge of a ruler is placed on the map so as to pass through a,
and is carefully aligned on A. Keeping the ruler in this position,
a line is ruled from a towards the observer. It is obvious that
the position of the observer must be somewhere along that line.
'The ruler is then adjusted, with its edge passing through band
aligned on B. A second line is drawn towards the observer from b.
The intersection of these two lines at x marks the position of the
observer.
(To be continued.)
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Notes on Military Map Reading
A. P. Blenkinsop
J R Army Med Corps 1910 14: 272-284
doi: 10.1136/jramc-14-03-04
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