Map Reading

Introduction

901. The purpose of this chapter is to refresh your memory of the subjects covered in map reading. (See The Manual of Map Reading, Field Sketching and Air Photo Reading, Part 1.)

902. It is an essential requirement of ALL soldiers that they be able to read and use a map well. This ability to read maps will only come with practice, so wherever possible, obtain a map and practice reading and using it. The more familiar you are with it, the greater will your ability be to use it.

903. It cannot be too greatly stressed that the essential requirements for a good map reading are CARE and ACCURACY. If you remember this and apply it to your map reading, you cannot help but to improve your ability. A list of definitions is included at the end of the chapter. Check this list if you strike a word whose meaning is not clear to you.

Units of Measurement

904. Map reading is essentially measuring and relating map to ground. We use certain units to do the measuring. They are:

a. Degrees. Most of you will remember measuring angles in degrees at school. There are 360 degrees in a circle (90 degrees in a right angle). For smaller angles each degree is divided into 60 minutes; each minute divides into 60 seconds. Although degrees are still used for some purposes the Army, in order to standardise with our Allies, is gradually converting to a unit of angular measurement which is called a "mil".

b. Mils. For practical purposes there are 6,400 mils in a circle; 1,600 in a right angle. One degree equals 17.77 mils (18 mils approximately). In approximations of small angles the mil can be taken as the angle subtended by one unit of length at a distance of 1,000 units, eg,

1 yard at 1,000 yards subtends 1 mil.

300 metres at 2,000 metres subtends 150 mils.

This relationship may be utilised for angles of 180 mils or less.
- (1) Conversion rules:
  - (a) Rough rule - Degrees X 20 = mils.
  - (b) Good approximation - Multiply degrees by 20 and subtract 11 percent.
  - Example:

90 degrees =	90 X 20	1,800
less 180 (10%)
less 18 (1%)=		198
		1,602

Thus the rule gives us 1,602 when the exact value is 1,600.

Metres. A metre is a yard plus 3½ inches.
- (1) 1000 metres = 1094 yards.
- (2) 100 metres = 109.4 yards.

Production of Maps

905. Modem maps are produced by a combination of vertical aerial photographs and accurate ground survey. They are produced to show as much as possible of the natural and artificial features on the ground.

Scale

906. The scale of a map indicates the ratios between any distance on the map and the corresponding distance on the EARTH's surface. By altering the scale of a map we can put it to many different uses. For instance the map which soldiers up to battalion level use is of sufficiently large scale to show as much detailed ground information as possible and does not cover a very large area whereas the map used at division or army HQ level must cover a large area so that troop movement can be planned at the expense of detail. Scale is given in several different forms. Some of the scale shown are on the maps on Pages 82 to 84.

Marginal Information

907. Because of the scale of the map, a lot of information cannot be included within the body of the map. This essential information is included in the margins of the map and is normally found in the locations indicated in the diagram below.

Orientation.

908. To identify features of the landscape with those on the map, it is necessary to turn the map so that the north point of the map corresponds with the direction of north on the ground. This can be done by:

a. Setting a map by inspection. Locate your position accurately. Line up a feature that you can recognize on the map, e.g. a straight road, with the same feature on the ground. Note that. if the road curves you must know exactly where you are on it or the setting may be badly out. At a cross roads you know your position exactly and you have four directions to set your map by. Be sure though that the right roads point in the right direction. If you are not sure, note the direction of some other object, a house or a church, and check that its direction on the map is correct. The basis of this method is that there should be something clearly marked, such as a road or railway, whose direction on the map can be made to coincide with the direction on the ground.

b. Setting a map by compass. If you do not know where you are, or if you are in jungle or featureless country, you will have to set the map by compass. You will be told more about the compass later in this chapter. The best way is to work out the magnetic variation from grid North; then lay the compass so that its axis lies along any North-South grid line and turn the map until the North point on the compass card points East or West of the lubber line by the amount of the variation. It might seem less trouble to draw in the magnetic North line on the map. It certainly involves less calculation but there are practical difficulties if you have already folded your map.

909. Here is a useful tip for keeping the map set after you have set it by compass. Pick out some distant object that is easily seen and remembered, it does not matter whether it is marked on the map or not. With the map set, line up a ruler on the object and rule in the line, This may be done on any convenient place on the map. So long as you do not move too far from your original position the map will always be set when the line points towards the object.

Relief

910. It is necessary to indicate natural features on a map by some system of relief so that we can see in advance from the map what the country looks Eke. Illustrated below are several methods in common use of showing relief:

a. Shading. Brown wash of various colour tones are used to show slopes of varying steepness. The darker the colour the steeper the slope. Height is shown by spot heights (0 567) scattered over the map. Normally used on small scale maps. (eg, 1:250,000).

b. Layer Tints. Used on small maps with contours where contour is 250 ft or greater. Different shades indicate different heights. The darker the shade, the greater the height

c. Contours. The most accurate and most common method of showing relief is by means of contour lines. A contour line is a line drawn on a map which passes through points on the earth's surface lying at the same height above sea level. Contour lines actually follow the shape of the ground and provide the only means of representing individual relief features precisely on a map. The diagram below shows how contour lines fit into the shape of the ground and show relief.

Slopes

911. Slopes are classified into three classes, each of which may be recognised on the map by the contours.

912. Uniform Slope. A uniform slope is one which does not vary in steepness over its whole length. On the map the contours will be evenly spaced. Such slopes may be steep or gentle.

913. Convex Slope. A convex slope is one which is steep at the lower levels, becoming less steep towards the top. In such a slope contours appear close together at the bottom of the slope and gradually widen out towards the top.

914. Concave Slope. The concave slope starts gently at the bottom and becomes steeper towards the top. In this type contours appear wider at the bottom and closer at the top.

915. Steepness. Where contours are close together, slopes are steep. The wider apart the contours, the gentler the slope.

916. Summary of Slopes. The following diagram shows various features as they are shown by contour lines on a map. Note that contour lines are marked at intervals by figures which show the height above sea level. The figures always face uphill.

917. The figures above illustrate various types of country as shown by contours. The consolidation below illustrates how, they fit together.

The Grid System of Reference
918. On all military maps, a grid system is provided to enable us readily to refer to any point
on a map.

919. The Grid. This consists of two sets of parallel lines (one vertical, the other horizontal).
These cross each other at right angles, thus forming squares. On Australian maps, depending
upon the scale, grid lines are spaced at an interval of 1000 yards for maps at scale of 1:50,000
whilst for maps at scale of 1:250,000 grid lines are spaced at 10,000 yards. Grid lines are
numbered horizontally and vertically.

920. Grid Reference - Four Figures.

a. Using the grid lines, a grid reference may be given by indicating the intersection
of any two lines. This is done by referring to the vertical lines first then the horizontal.

b. To indicate the intersection circled in the sketch below, the figures 83 (vertical line)
and 14 (horizontal line) would be used and expressed as: GRID REFERENCE 8314.

c. This is known as a FOUR FIGURE REFERENCE and is normally used only to indicate a 1000 yards/metre square, of which the intersection is the SW corner. (In this case, square A.) The grid reference 8415 would indicate the south west corner of the 1000 yards/metres square shown divided into 100 smaller squares.

921. Grid References- Six Figures.

a. If it is desired to identify some point within a 1000 yards/metres, a more accurate reference must be given. This is done by mentally dividing the sides of the square into ten divisions and numbering these lines from the SW comer. (Bottom left.)

b. To indicate the house shown near the intersection of the sixth vertical line and the fifth horizontal line, the numbers of these lines are placed after the numbers of the 1000 yards/metres square. In this case the reference of the house is: 846 155.

Note that we use the same drill, i.e. work from SW corner.

GIVE VERTICAL (EASTINGS) FIRST AND HORIZONTAL (NORTHINGS) NEXT.

c. Remember that the 100 yard division lines are not printed on the map. You must visualise these or use the scale on a protractor.

THE COMPASS

Description

922. The compass is an instrument we use to find angles and bearings on the ground. The following diagram shows the main component parts of the service compass:

923. Note that this compass is graduated in both mils and degrees. For most purposes it is now normal to use the mil scale. Of course you may still strike compasses which are only graduated in degrees but most of these are gradually going out of service. Irrespective of the unit of graduation the method of using the compass varies little.

Use of a Compass

924. To use the compass accurately, it must be held correctly and the correct sight picture obtained, as shown here:

925. Because the compass is a magnetic instrument it is important that we observe certain rules and take precautions against inaccuracy when. we use it. Metal attracts the compass needle, therefore when using a compass, keep the distance shown away from these objects:

a. Equipment	4 metres
b. Tank (AFV)	80 metres
c. Transmission HT line	80 metres
d. Fencing Wire	10 metres
e. Large metal objects (eg, field gun)	60 metres

Setting the Compass for Night Marching

926. The following are the steps to be taken to set the compass for night marching:

a. Lay the lid open flat with the luminous North Point underneath the black lubber line.

b. Rotate the glass face until the luminous strip on the face is reading the reverse bearing to that required. To obtain your reverse bearing, simply subtract your required bearing from 6,400 mils (or 360 degrees).

c. Hold the compass steady in the centre of the body with the lid away from you.

d. Rotate the whole body around until the luminous diamond or North Point on the card is directly beneath the luminous strip on the glass face.

e. Your required direction or bearing is now indicated by the direction of the hair line on the lid and the black lubber fine.

THE PROTRACTOR

Use
928. In using the protractor, remember accuracy is the important factor and accuracy will be greatly increased if the pencil you use is kept very sharp. Heavy pencil lines can throw a reading out by anything up to 24 mils or 1.5 degrees. KEEP YOUR PENCIL SHARP! It may mean the difference between finding or missing a minefield gap! Illustrated below is the method of using a protractor to find an angle or bearing.

MISCELLANEOUS

Conversion of Bearings
929. When we find a bearing on a map with a PROTRACTOR, we call it a GRID BEARING. A bearing taken with a COMPASS is called a MAGNETIC BEARING. In most places on the earth's surface, there is a difference between the two called the MAGNETIC VARIATION. (See MARGINAL INFORMATION on the map.) If we wish to march along a bearing which we have plotted on a map, we must convert it to a MAGNETIC BEARING. To do this conversion, a simple diagram is all that is necessary.

Note: When converting from GRID to MAG or MAG to GRID, ALWAYS draw a diagram like that above; draw it - on the ground if nothing else is available. Always read a bearing in a clockwise direction from North.

930. The magnetic variation is given in the marginal information both in MILS and DEGREES. It varies slightly each year. Make sure you set the up-to-date variation.

Measuring Distances

931. To measure distances between two points on a map which are in a straight line, we mark the distance onto a straight edge and measure it against a linear scale. When measuring distances between points joined by a winding route, the same principle applies. Straighten out the route first by following it with a piece of cotton or marking each straight stretch on a piece of paper and measure the distance so found against the linear scale.

Resection 932. If you are unable to locate your position on the map by comparing the detail on the
map with the ground, then you must find your position by resection. 933. To do this:

a. First identify three points on the ground that you can locate on the map. Two I suffice in an emergency.

b. Then take bearings to these points on the ground with a compass. Next convert these magnetic bearings to grid bearings and then to back bearings.

c. Using a service protractor, plot the back bearings from the identified points on the map, and draw in the bearing lines. These lines will cross. The point where they cross is your position.

d. If the bearing lines db not cross or almost cross at the one point then inaccuracy in taking or plotting bearings is indicated. Do it again.

Back Bearings

934. Apply the following simple rule:

a. If bearing is less than 3200 mils, add 3200 mils.

b. If bearing is more than 3200 mils, subtract 3200 mils.

c. The result is back bearing.

Finding North without a Compass

935. North may roughly be found by using your watch by day and the stars by night: a. Using your Watch:

(1) When South of the equator lay the watch with twelve o'clock pointing to the sun. The direction of North lies midway between the hour hand and twelve o'clock.

(2) When North of the equator lay the watch with the hour hand pointing to the sun. The direction of North lies midway between the hour hand and twelve o'clock. b. Using the Stars:

(1) When South of' the equator the Southern Cross is the most useful and easily recogpised constellation. To find the direction of South extend the longer arm of the cross (which is the shape of a kite) four and one half times in the direction of the tail. The point reached is due South.

(2) When North of the equator, the Pole star indicates the direction of North.

GLOSSARY OF TERMS

Contour. An imaginary line on the surface of the ground at the same height above mean sea level throughout its length. Contour lines are drawn on maps to show the shape of the ground. Control Point. A point fixed during the initial survey of an area of ground; marked on Ordnance Survey maps by a small triangle, with the height.

Crest. The highest point of a hill or mountain range. That line on a range of hills or mountains from which the ground slopes down in opposite directions.

Datum or Datum Level. The level from which altitudes are measured. Generally mean sea level.

Detail. All the topographical information on the map.

Divide. The line along a range of hills from which the water flows in opposite directions, eg, the Great Dividing Range.

Escarpment. The steep hillside formed by a sudden drop in the general ground level, usually from a plateau.

Gorge. A narrow stream passage between steep rocky hills; a ravine with precipitous slopes.

Grid. Parallel vertical and horizontal lines forming squares drawn on a map as a basis for a system of map references.

Knoll. A small knob-like hill.

Local Magnetic Attraction. Attraction due to the presence of iron or magnetic iron ore which causes a compass needle to deviate from its true position.

Orienting. A map. Placing a map so that the direction of North on the map conforms with the direction of North on the ground. Also called "Setting a Map".

Plateau. An elevated region of considerable extent, generally fairly level.

Ravine. A long deep valley worn out through the centuries by a river or stream.

Re-entrant. A valley or ravine, usually between two spurs, running towards the hill or mountain
top.

Resection. A method of fixing a position by observation of at least three previously fixed
points.

Plotting. The process of transferring to a map or sketch, observations, bearings and measurements taken in the field.

Ridge. The line along a hill or range of mountains or hills from which the water flows in opposite
directions; a divide; sometimes the crest of a line of hills as it appears along the horizon.

Saddle. A depression between two hills or mountains.

Spot Height. A point on a map whose height has been determined by field survey methods. This height is printed beside the point on the map.

Spur. A minor feature, generally in the form of a ridge, running out from a hill or mountain.

Vertical Interval (VI). The difference in level, measured vertically, between adjacent contours AF
on a map.

Watershed. The line separating the water flowing into two different river systems; the edge of a
water basin.