Fundamental OS Map Skills: Scale, Coordinates and Relief

Locating places and understanding the land

A topographical map represents the position of features and the shape of the land. Three parts of the map work together: coordinates tell you where something is, scale lets you calculate how far away it is, and height information shows how the land rises and falls. The key explains the symbols used for individual features.

These skills apply to OS maps at both 1:25 000 and 1:50 000, as well as other maps with suitable grids, scales and height information. Always read the information printed on the particular map rather than assuming every map uses the same conventions.

Four-figure grid references identify a square

A map grid provides a coordinate system. The numbered vertical lines are eastings: their values increase as you move east across the map. The numbered horizontal lines are northings: their values increase as you move north up the map.

Read the easting first, then the northing. For a four-figure reference, find the vertical line immediately to the left of the feature and the horizontal line immediately below it. Together, these identify the square’s bottom-left corner.

For example, a feature between eastings 17 and 18 and northings 51 and 52 lies in grid square 1751. This reference identifies the whole square, so several different features can share it.

Six-figure grid references locate a point more precisely

To locate a point within that square, imagine dividing the square into ten equal parts across and ten equal parts upwards. Add a third digit to each coordinate to show the point’s position within the square.

Suppose a point in square 1751 is three tenths of the way across from easting 17 and six tenths of the way up from northing 51. Its easting is 173 and its northing is 516, giving the six-figure reference 173516.

When using a real OS extract, apply the same method to its printed grid lines. The diagram is a constructed demonstration, not a map of a real place. A six-figure reference gives greater precision than a four-figure reference, but still depends on how accurately you estimate the tenths.

Scale connects map distance to ground distance

A ratio scale compares distances using the same units. At 1:25 000, one unit on the map represents 25 000 of those units on the ground. Therefore, 1 cm represents 25 000 cm, which is 250 m. At 1:50 000, 1 cm represents 500 m.

For a straight-line distance of 4 cm on a 1:25 000 map, multiply 4 by 250 m. The ground distance is 1 000 m, or 1 km. The same 4 cm measurement on a 1:50 000 map represents 2 km. The map measurement alone is therefore not enough: its scale matters.

Measuring straight and curved distances

For a straight-line distance, place a ruler between the two locations, measure in centimetres and convert using the map’s scale. Alternatively, mark the two endpoints on the edge of a piece of paper and compare the gap with the map’s scale bar.

A winding road or river needs a different approach because its route is longer than the direct distance between its endpoints. Divide it into short, approximately straight sections. Measure each section and add the measurements before converting the total to a ground distance.

You can also use a paper edge: mark the first section, pivot the paper at the bend, and continue marking successive sections along the same edge. Compare the accumulated length with the scale bar. Shorter sections follow a tightly bending route more closely; long sections cut across bends and underestimate its length.

Contours and spot heights represent relief

Relief means the height and shape of the land. A spot height gives the height at one particular marked point. A contour line joins points of equal height, so every location along a contour labelled 100 m has that height.

The contour interval is the vertical difference between neighbouring contours. Contours commonly use intervals of 5 m or 10 m, but check the map’s information. With a 10 m interval, successive contours might be 100 m, 110 m and 120 m. This interval is a difference in height, not the horizontal distance between the lines on the page.

Spot heights and contours complement one another: a spot height provides a value at a precise location, while contours reveal the shape of the surrounding slopes.

Reading slope steepness and landforms

The gradient describes how steep a slope is. Where contours are close together, the land changes height over a short horizontal distance: the slope is steep. Where the same contour interval is spread over a greater horizontal distance, the slope is gentler.

Compare the two panels by following a line across the contours. Both show the same increase in height, but the closely spaced contours show that increase occurring over less ground distance. In a real map comparison, check both the contour intervals and the map scales before judging steepness.

Contour patterns also help identify landforms. Nested, roughly circular contours with heights increasing towards the centre indicate a hill. V-shaped contour patterns can indicate a valley. Read the height labels as well as the pattern to establish how the land rises or falls.

Calculating gradient from height and distance

Contour spacing gives a visual impression of steepness; calculating gradient lets you express it numerically. First find the vertical height difference by subtracting the lower height from the higher height. Then measure the horizontal distance between the points on the map and convert it to a ground distance using the scale.

Use the same units for both distances, usually metres:

$$\text{gradient} = \frac{\text{vertical height difference}}{\text{horizontal ground distance}}$$

Gradient can be expressed as a ratio 1:n. Divide both sides of the height-difference-to-distance ratio by the height difference, so that the first number becomes 1. Equivalently, calculate $n = \text{horizontal ground distance} \div \text{vertical height difference}$.

For example, suppose point A is at 100 m and point B is at 200 m, and they are 3 cm apart on a 1:50 000 map. The height difference is $200 - 100 = 100$ m. At this scale, 1 cm represents 500 m, so the horizontal ground distance is $3 \times 500 = 1\,500$ m.

The gradient is therefore $100 \div 1\,500 = 1/15$. As a ratio, $100:1\,500$ simplifies to 1:15. This means the land rises by 1 m for every 15 m travelled horizontally from A towards B. It describes the average gradient between the points; individual parts of the slope may be steeper or gentler.

A gradient of 1:10 is steeper than 1:20: the same 1 m rise occurs over a shorter horizontal distance. This is the numerical version of the close-contour pattern.

Applying the idea to other isoline maps

Contours are one type of isoline: a line joining points with an equal value. Other isoline maps represent different quantities rather than land height.

The reading method transfers: identify the quantity and its units, read the line values, check the interval, and examine the spacing. Closely spaced isolines show a rapid change in the mapped quantity over distance. Only when the lines represent height does this spacing directly describe the steepness of the land.

Coordinates

• Eastings first, northings second.
• Four figures: identify a square using its bottom-left corner.
• Six figures: add estimated tenths across and upwards to locate a point more precisely.

Scale and distance

• 1:25 000: 1 cm represents 250 m.
• 1:50 000: 1 cm represents 500 m.
• Straight distance: measure between endpoints and convert using the scale.
• Route distance: measure successive short sections, total them, then convert.

Relief and gradient

• Contour: equal height along the line.
• Contour interval: vertical height difference between neighbouring lines.
• Spot height: height at one marked point.
• Close contours → steep slope; widely spaced contours → gentle slope.
• Compare contour intervals and map scales when comparing steepness.
• Nested contours increasing in height towards the centre → hill; V-shaped contours can indicate a valley.

$$\text{gradient} = \frac{\text{vertical height difference}}{\text{horizontal ground distance}}$$

Use the same units. For 1:n, calculate $n = \text{horizontal distance} \div \text{height difference}$. Smaller $n$ → steeper slope.

Other isolines

Read the mapped quantity, units and interval. Close spacing means rapid change over distance, not necessarily steep land.