NOTES ON MILITARY MAP READING. (Continued ft01n p. 49., vol. xiv.)

Transcription

1 272 NOTES ON MLTARY MAP READNG. By MAJOR A. P. BLENKNSOP. Royal Army Medical Corps. (Continued ft01n p. 49. vol. xiv.) TRUE AND MAGNETC NORTH. T 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 Sgn (fig. 9) which indicates a magnetic variation of 15 W. FG. 9. " The variation is subject to two principal changes annual and posi tional." " Annual Ghange.-n London the variation was 11 E. in t 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." n 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

2 A. P. Blenlcillsop 273 the position on the earth's surface. n northern ndia the variation is about 2 E. while in the extreme south the needle points nearly due north. n South Africa there is a variation of about 25 V. and in France and Germany from 10 to 15 W." n 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. n 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. n England the error from this cause would seldom be more than o 180 ~ 0) 0 FG. 10. " / "is-..?o ~o 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. t 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 90 "0 '<? o

3 274 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. t 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 ) UtO + l!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. FG. 11. Magnetic bearing is the angle a line makes with the magnetic north line. n 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

4 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. t 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 !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 = - 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 io = 365io 365! = 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 FND THE DRECTON 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

5 276 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 38i o north at midsummer and 38i o 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." FG. 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. f 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

6 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:- "n 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 X. and the pointer of the hour hand is approximately a south line." "n the Southern Hemisphere.-Hold the watch as before but in this case point the line from the centre of the dial to figure X. 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. t 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 X. 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) n 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." " n order to identify tbe North Star note the following diagram (fig. 13) of the seven stars of the constellation known as the' Great Bear.' "

7 278 * Notes on Milita171 Map Reading N p. * ' s* * * FG. 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 " s" in the tail of the "Great Bear." (b) n 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 1 0 of the South Pole. f 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. 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:-' *

8 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. t 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 ' nformation 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. SETTNG 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

9 280 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. f 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. t 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 0 N 360orO 270 W '-:'~----- l 90 0 'P... // "'" " '" o S 180 FG. 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

10 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 SETTNG A MAP. A. With a Oompass.-f 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. f 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. f 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) dentify 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). (nstead 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.)

11 282 Notes on Military Map Reading... '.. "" FG. 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. ts 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. f 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

12 A. P. Blenlcinsop 283 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 :FND ONE'S POSTON ON THE MAP. t 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. t 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). " OB?C ~b :c Tracing paper x FG. 16.

13 284 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. f 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 0A x FG B 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. t 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.)