Map Reading

Main Idea

Map reading and navigation involve interpreting a scaled, symbolic representation of the ground to determine locations, distances, directions and physical features. Accurate navigation depends on using scale, conventional signs, grid references, bearings and contour information together with a compass, pacing, timing and observation of the surrounding terrain.

Key Concepts and Definitions

• Map: A reduced and conventional representation of a part of the earth's surface on a flat sheet.
• Scale: The relationship between a distance on the map and the corresponding distance on the ground.
• Representative Fraction: A scale written as a ratio, such as 1:50,000, where one unit on the map represents 50,000 of the same units on the ground.
• Conventional Signs: Standard symbols used to show features such as roads, railways, buildings, rivers, bridges, vegetation and boundaries.
• Grid: A network of numbered lines printed on a map to help locate positions accurately.
• Eastings: Vertical grid lines whose numbers increase from west to east.
• Northings: Horizontal grid lines whose numbers increase from south to north.
• Grid Reference: A numerical method of identifying a particular position on a map by using eastings and northings.
• Four-Figure Grid Reference: A reference identifying a grid square, normally given by reading the easting first and the northing second.
• Six-Figure Grid Reference: A more precise reference identifying a point within a grid square by estimating tenths between the relevant easting and northing lines.
• Direction: The position of one point from another, commonly expressed using the cardinal and intermediate points of the compass.
• Bearing: The horizontal angle from north to a selected direction, measured clockwise from 000 degrees to 360 degrees.
• Grid Bearing: A bearing measured from grid north shown by the map's grid lines.
• Magnetic Bearing: A bearing measured from magnetic north using a magnetic compass.
• Magnetic Variation: The angle between grid north and magnetic north at a particular place and time.
• Compass: An instrument with a magnetic needle used to find magnetic north and follow bearings.
• Contour Line: A line joining points of equal height above mean sea level.
• Contour Interval: The constant vertical difference in height between two successive contour lines.
• Spot Height: A marked point showing the exact or approximate elevation of a particular location.
• Gradient: The steepness of a slope, expressed as the vertical rise compared with the horizontal distance.
• Ridge: A long, narrow area of high ground, often with slopes falling on both sides.
• Valley: A long depression or low area between higher grounds, often containing a stream or river.
• Saddle: A low gap or depression between two higher points.
• Re-entrant: A small valley or indentation projecting into higher ground.
• Spur: A finger-like projection of higher ground extending from a ridge or hill.
• Intervisibility: The condition in which two points can be seen from each other without an obstruction.
• Route Selection: Choosing a practical path by considering distance, slope, obstacles, water crossings, visibility and safety.
• Map Orientation: Placing the map so that its directions and features correspond correctly with the ground.
• Pacing: Estimating distance traveled by counting a person's normal walking steps.
• Dead Reckoning: Estimating present position by using a known starting point, direction, distance and time traveled.

Supporting Arguments and Evidence

• Directions are expressed using the four principal points—north, east, south and west—and the intermediate directions northeast, southeast, southwest and northwest. A bearing is written as three figures: north is 000 or 360 degrees, east is 090 degrees, south is 180 degrees and west is 270 degrees.

• The back bearing is calculated by adding 180 degrees when the original bearing is less than 180 degrees, and subtracting 180 degrees when the original bearing is 180 degrees or more. Grid bearings use grid north, whereas magnetic bearings use magnetic north; the difference between them is magnetic variation.

• Grid references must be read by going right to the easting first and then upward to the northing. Four-figure grid references identify a grid square, while six-figure grid references identify a substantially more precise point within that square.

• Scale allows map distances to be converted into ground distances. When a representative fraction is used:

Ground distance = map distance × scale denominator

On a 1:50,000 scale map, 1 centimetre represents 50,000 centimetres, or 500 metres, on the ground.

• Conventional signs and marginal information provide essential map information. A map commonly includes its title, scale, north direction, legend, grid system, contours, marginal information and the date or source of survey. Symbols may represent roads, railways, buildings, rivers, bridges, vegetation and boundaries.

• Contour lines provide a three-dimensional understanding of terrain on a two-dimensional map. Closely spaced contours indicate a steep slope, widely spaced contours indicate a gentle slope, and equally spaced contours indicate a uniform slope. A convex slope has contours widely spaced near the top and closely spaced near the bottom, while a concave slope shows the opposite pattern.

• Closed contours generally indicate a hill when heights increase inward. Depressions may be represented by inward-pointing hachures or decreasing heights. Contour lines normally do not cross, except in unusual landforms such as an overhanging cliff.

• Contour patterns assist in identifying landforms. Contours cross streams and valleys in a V-shape, with the V generally pointing uphill or toward the source of the stream. Ridges, valleys, saddles, re-entrants and spurs can therefore be interpreted from the arrangement and shape of the contours. Gradient is calculated as:

Gradient = Vertical interval ÷ Horizontal distance

• Intervisibility determines whether two points can be seen from one another without an obstruction. This is relevant to route selection, which must consider slope, obstacles, water crossings, visibility, accessibility and safety. The safest and most efficient route is therefore not always the straightest route.

• Map orientation requires the map to be placed so that its directions and features correspond with the ground. To orient a map with a compass, place the compass on the map, align it with the north-south grid lines, and rotate the map until the magnetic direction is correctly related to the map.

• A compass should be kept away from metal objects, electrical equipment and magnetic materials that may affect the needle. Accurate navigation requires attention to magnetic variation and to the distinction between grid north and magnetic north.

• Route planning should identify the starting point, destination, intermediate checkpoints, direction, distance, terrain, obstacles, rest points and alternate routes. Position should be confirmed regularly by comparing the map with visible ground features such as hills, roads, streams, buildings and ridgelines.

• Distance estimation may involve map measurement, pacing, timing, known landmarks and GPS or other electronic aids where permitted. Dead reckoning uses a known starting point, direction, distance and time traveled to estimate the present position.

• Navigation errors may result from confusing similar features, misreading grid references, using the wrong north, ignoring magnetic variation, losing count of paces or failing to update the position. Accurate navigation therefore requires preparation, consistent measurements and correction of errors caused by scale, magnetic variation or terrain.

What to Remember

Map reading combines scale, conventional signs, grid references, bearings, contours and field observation to locate positions and interpret terrain. Read eastings before northings, distinguish grid north from magnetic north, and use contour spacing and shape to assess slopes and landforms. Effective navigation depends on orienting the map, checking position at regular checkpoints and selecting routes according to terrain, obstacles, visibility, water and safety rather than distance alone.