Ordnance Survey maps and map reading

Overview

Four- and six-figure grid references, what a scale of 1:50 000 means on the ground, how contours show relief, and choosing the right map for a piece of fieldwork.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   How do you give a four-figure grid reference, and what does it identify?

   Model answer

   Read the two-digit number of the easting line along the bottom of the map first, taking the line to the left of the square, then the two-digit number of the northing line up the side, taking the line below the square, and write them as a single four-figure number such as 3247. It identifies a whole grid square, which on a standard Ordnance Survey map represents one square kilometre on the ground. The order is remembered as along the corridor and then up the stairs - eastings always before northings.

2. 2

   How do you give a six-figure grid reference, and how precise is it?

   Model answer

   Take the four-figure reference for the square, then mentally divide the square into tenths in both directions. Estimate how many tenths east the feature lies from the left-hand easting and insert that digit after the easting pair, then estimate how many tenths north it lies from the bottom northing and insert that digit after the northing pair. Where a grid square is one kilometre across, a six-figure reference locates a feature to the nearest hundred metres, which is precise enough to identify an individual fieldwork site.

3. 3

   What does a scale of 1:50 000 actually mean, and how does it convert to distance on the ground?

   Model answer

   It means that one unit of length on the map represents fifty thousand of the same units on the ground. One centimetre on the map is therefore fifty thousand centimetres, which is five hundred metres, so two centimetres represent one kilometre. Because the grid squares represent one kilometre, each square measures two centimetres on the map.

4. 4

   How does a 1:25 000 map differ from a 1:50 000 map, and when would you choose each for fieldwork?

   Model answer

   At 1:25 000 one centimetre represents two hundred and fifty metres, so four centimetres represent one kilometre and the grid squares are four centimetres across, twice the size of those on a 1:50 000 map. The larger scale shows far more detail, including field boundaries, footpaths and individual buildings, so it is the right choice for planning and recording sites within a single study area. The 1:50 000 map covers four times the area on the same sheet, which makes it better for showing the wider regional setting, for planning routes between sites and for identifying the drainage basin or settlement network as a whole.

5. 5

   What is a contour, and what is a contour interval?

   Model answer

   A contour is a line on a map joining all points of equal height above sea level, so it is a type of isoline. The contour interval is the fixed vertical difference in height between one contour and the next - commonly ten metres on a 1:50 000 map, and five or ten metres on a 1:25 000 map depending on the terrain. Because the interval is constant, it is the horizontal spacing of the lines that tells you about the slope.

6. 6

   What does the spacing of contours tell you about the land?

   Model answer

   Contours drawn close together mean a large height change over a short horizontal distance, which is a steep slope; contours spaced widely apart mean the same height change is spread over a long distance, which is a gentle slope. Contours that merge or almost touch indicate a cliff or a very steep face, and an area crossed by no contours at all is flat or very gently sloping.

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