B. Map reading and navigation

Map reading and navigation are essential skills for mission personnel and have three specific purposes in the context of crisis management missions. These are to:

1. Navigation aids

Maps

Maps are the most important navigation aid and should be studied carefully as a preliminary to cross-country navigation. Doing so can provide the answers to many questions, such as the best route to be taken and areas to be avoided. Maps enable the user to visualise the lie of the land, assist with sense of direction and increase confidence.

Topographic maps are detailed graphic representations of features that appear on the earth’s surface. A map’s legend (or key) lists the features shown on the map and their corresponding symbols. Topographic maps usually show a geographic graticule (latitude and longitude in degrees, minutes and seconds) and a coordinate grid (eastings and northings in metres) so that relative and absolute positions of mapped features can be determined.

How to read a topographic map

The first step in reading a topographic map is to become familiar with the specific characteristics of the map or maps that are being used, such as:

Map scale

A map represents a given area on the ground. A map scale refers to the relationship (or ratio) between distance on a map and the corresponding distance on the ground. Map scales can be shown using a scale bar.

Using a 1:250,000 scale map, for example, the first number of the scale (1) represents a core unit of distance on the map while the second (250,000) represents that same distance on the ground. In this case, one centimetre on the map represents 250,000 centimetres, or 2.5 kilometres, on the ground.

The scale bar can be used to determine the distance between two points on the map. Scales are usually shown in increments of one, five or ten kilometres. Use a piece of string, a ruler or a strip of paper to measure the distance between two points on the map. Then compare that measurement to the scale bar on the map to determine what distance the measurement represents.

Direction and bearings

Maps usually include a north point diagram that shows the direction of true north, grid north and magnetic north. This diagram also shows the actual grid/magnetic angle for the centre of the map face.

The angular difference between TN and MN is known as magnetic declination. As GN is used in preference to TN for map reading purposes, it is more useful to know the difference between GN and MN. This is known as the grid/magnetic angle or magnetic variation. As the position of the north magnetic pole moves slightly from year to year, the grid/magnetic angle and magnetic declination will vary by a small amount each year. In using a map for accurate navigation, magnetic variation can be important, particularly if the map is several years old.

Directions can also be expressed as bearings. A bearing is the clockwise horizontal angle, measured from north to a chosen direction. Bearings are usually shown in degrees and range from 0° (north) to 360° (also north). South is 180°, east is 90° and west is 270°.


Illustration depicting bearings of 40° and 320°.

Map symbols (the legend)

Maps use symbols to represent features on the ground. These features include roads, tracks, rivers, lakes, vegetation, fences, buildings, power lines, administrative boundaries, etc. Colour plays an important part in symbols and some international conventions apply to the use of colour. For example, blue for water features, black for culture and green for vegetation. While most symbols are easily recognised as the features they represent, they are all explained in the map’s legend.

Contour lines

Topographic maps show contour lines that join points of equal height and represent the relief in the terrain depicted. For example, if there are many contour lines close together, the terrain is steep. Contour lines that are far apart indicate land with gentle slopes.

Datums

Mapping and coordinate systems are based on a datum, which is a mathematical surface that best fits the shape of the Earth. A geocentric datum is a datum that has its origin at the earth’s centre of mass. The advantage of the geocentric datum is that it is directly compatible with satellite-based navigation systems.

Adopting a geocentric datum allows for a single standard of collecting, storing and using geographic data, which ensures compatibility across various geographic systems at the local, regional, national and global level.

Anyone using a map or a GPS receiver will need to know which datum is being used for the grid and the latitude and longitude coordinates.

2. Map coordinates

Map coordinates are usually shown in one of two ways: geographic coordinates or grid coordinates.

Geographic coordinates: latitude and longitude

You can find or express a location using the geographic coordinates of latitude (north or south – horizontal lines) and longitude (east or west – vertical lines). These are measured in degrees (°), minutes (’) and seconds (”). For example, the geographic coordinates for a position could be stated as 33°40’30”S, 153°10’40”E. Each degree is divided into 60 minutes and each minute is divided into 60 seconds.

Latitude is the angular expression of the distance north or south from the equator (0° latitude). The South Pole is at 90°S; the North Pole at 90°N. Longitude is the angular expression of the distance east or west of the imaginary line known as the Prime Meridian (0° longitude on all maps).

Latitude and longitude coordinates are shown at each corner of a map’s face. On some maps, short black lines along the edges of the map face indicate the minutes of latitude and longitude. When expressing coordinates, latitude is given first.

Grid coordinates: eastings and northings

Grid lines can also be used to find or express a location. Grid lines are the equally spaced vertical and horizontal intersecting lines superimposed over the entire map face. Each line is numbered at the edge of the map face. On 1:100,000 scale maps, the distance between adjacent lines represents 1,000 metres or 1 kilometre.

Maps are normally printed so grid north points to the top of the sheet (when the print is the normal way up). One set of grid lines runs north-south while the other set runs east-west. The position of a point on the map is described as its distance east from a north-south line and its distance north of an east-west line. For this reason, grid lines are also called:

The squares formed by intersecting eastings and northings are called grid squares. On 1:100,000 scale maps, each square represents an area of 100 hectares or one square kilometre.

How to quote a grid reference for a particular point

A grid reference is used to describe a unique position on the face of the map. The degree of accuracy required will determine the method used to generate a grid reference. All methods follow a similar approach. A four-figure grid reference is used to identify which grid square contains a map feature. A six-figure grid reference will further specify the position to an accuracy of one tenth of the grid interval. In a map’s margin there is usually a section devoted to how grid references are quoted. The information needed to complete a grid reference will be found in this section of the margin.

To obtain a complete 1:100,000 scale grid reference for point A (Panoro) on the map above:

3. Compass

The compass is a valuable aid to navigation, particularly when travelling at night or through dense vegetation where it is difficult to identify landmarks.

A compass works on the principle that the pivoting magnetised needle (or the north point of the swinging dial) always points to the north magnetic pole. As a result, a compass with graduations (degrees) marked on it can be used to measure the bearing of a chosen direction from magnetic north. Metal objects such as cars, fence posts, steel power poles and transmission lines can affect the accuracy of a compass reading. Stand clear of such objects when using a compass – at least one metre from metal fence posts and up to 20 metres from a car.

Always make sure to hold the compass level during use. Otherwise, the magnetic needle may jam in the casing.

Features of a compass

There are numerous types of compasses. The pivoted needle compass with an adjustable dial is the most useful type. In addition to a north-pointing needle, such compasses often have a transparent base with a direction-of-travel arrow and orientating lines marked on the rotating dial so they can be used for measuring grid bearings on a map.

Using the compass to reach a destination

To follow compass bearings to a chosen destination, either determine magnetic bearings from visible features along the route or obtain these bearings from another source prior to travelling.

To determine magnetic bearings:

When magnetic bearings are known:

4. Global Positioning System (GPS)

The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these satellites as reference points to calculate positions accurate to a matter of metres.

GPS receivers are generally hand-held devices that assist with navigation on the ground, at sea and in the air. The GPS receiver is only an aid to navigation and cannot be solely relied upon to navigate. It relies on the accuracy of the navigational data entered into the receiver.

How GPS works

The basis of GPS is triangulation from satellites. To triangulate, a GPS receiver measures distance using the travel time of radio signals. Using the signals from any three of these satellites, a two-dimensional position is given; using any four satellites, a three-dimensional and therefore very precise location of the GPS receiver is given.

What GPS can do

Some general functions of most GPS receivers include: determining ground speed, plotting current position, storing the current position as a waypoint, storing other positions as waypoints, plotting routes travelled, calculating a bearing between two positions, determining an error left or right of the intended track and determining a range or distance between two positions.

GPS navigation

Navigation with a GPS receiver is similar to navigation with a compass in that a map is used with both methods and a clear understanding of the principles of map reading and navigation is essential. Similar techniques to those used with map and compass navigation are used with GPS navigation. The principles of planning the intended route, studying the map, developing navigation data sheets, etc., still exist when using GPS receivers.

Using GPS with a map

GPS is based on the WGS84 datum (see explanation of datums above).

However, not all maps have a WGS84 datum. It is important to check which datum the map being used is based upon. Datum information will be shown in the map margin.

For the best match between coordinates of the map and GPS receiver, configure the GPS receiver to display coordinates (geographical or grid) on the same datum as the map being used. Most GPS receivers have the ability to display either geographic or grid coordinates on a number of national and regional datums. It is important to know how to set the correct datum in the receiver. Please consult the GPS receiver’s user guide for details. If the datum needed is not offered in the receiver, consult the relevant unit (e.g. GIS) in your mission for assistance.

It is recommended practice to check the GPS receiver against well-defined map features every time it is used. Visit a feature such as a road intersection, determine its position by GPS and compare this with coordinates calculated from a map. The larger the scale of this map, the better.

GPS performance and limitations

Most GPS receivers need to have a clear, uninterrupted view of the sky to enable communication with the satellite constellation (network). Some conditions that may interfere with GPS performance are:

Cloud cover, vegetation, operating inside a building, operating inside a motor vehicle without an external GPS antenna, operating in gorges, caves, mines and other underground or low ground areas. GPS receivers can also be affected by electrical storms.

Most commercial GPS receivers are accurate to an average of 50 metres horizontally and 70 metres vertically. As batteries power GPS receivers, it is important that the duration and condition of the batteries are known, particularly before heading into rural or remote areas. Spare batteries should be carried, but as a backup to the GPS receiver, navigators should ensure that they have a magnetic compass and map with them at all times.

Types of data that can be collected using GPS

There are two basic types of data that can be collected and stored in the memory of the GPS unit. These are waypoints (or points) and track logs (or tracks).

Waypoints (WPs) are a record of a specific point on the ground that has been visited. Track logs are a record of a series of mini-points that are collected automatically by the GPS during a journey.

How to use GPS to collect data

Here are some suggestions for using GPS to collect data in various situations on missions:

Remember that the GPS only records the WP numbers: a record of what these points represent must also be made (these are called the attributes). The attributes can be recorded in the device itself, in a notebook or on a form designed for this purpose.

5. Planning

Prior to travelling, the chosen route should be divided into legs. Each leg should end at an easily recognisable landmark. Then produce a navigation data sheet for the entire route, which gives significant information for each leg of the route.

Orientating a map

Orientate the map before reading it. To do this, hold the map horizontally and rotate it until its direction and features correspond to what is seen on the ground. If unable to identify the surrounding features, use the compass to orientate the map. To do this:

Once the map is orientated, prominent features in the landscape can be identified.

Finding the present position

A GPS receiver can be used to determine the coordinates once it is set to a datum corresponding to the datum on the map. Alternatively, once surrounding features on the ground and on the map have been identified, the following procedure can be used to find the current position:

Setting a course

Once the map has been orientated and the present position identified, a course can be set. Do this by sighting or by laying a straight line (using the edge of the map card or a piece of string) across the map. It is also good practice to identify a distant visible feature that is on the line, such as a rocky outcrop, and proceed to that point. Then identify another feature on the line, and so on, until the destination is reached.

When features are sparse, a GPS receiver can be used. First, determine the coordinates of the destination point from the map and enter them into the receiver. Then walk in the approximate direction of the destination, letting the receiver indicate the right direction.

Alternatively, the map and compass can be used as follows:

Maintaining direction using a compass

When moving through dense vegetation, it is important that continuous checks be made using the compass. The best method of maintaining a given magnetic bearing is to select a prominent object (such as a tree), which lies on the bearing, and move to it. Then select another object on the bearing and move to that. Continue with this method until the destination is reached. If it is impossible to find a prominent object on the bearing, then send another person forward about 100 metres, correct them onto the bearing and then proceed to them. Again, repeat this procedure until the destination is reached.

Once a course commences, checking must be continuous:

Distance travelled

It is very important, particularly when moving through vegetation, to know the distance that has been covered. There are two basic methods of achieving this: