A contour is an artificial line that connects points of equal height above the height datum, which is currently mean sea level, as recorded at Malin Head. The extract from the 1:50 000 Discovery Series below shows contours at 10 metre intervals, with spot heights at prominent points also given; Meendoran Hill reaches a height of 155m, while Ballintlieve Hill stands at 227m.

Contours tell us a lot about the gradient; note how the contours at Meendoran Hill are spaced widely, in comparison to Ballintlieve Hill. This indicates that the latter is much more steep than the former. Also, notice how the stream flowing north east from Ballintlieve Hill into Lough Fad has eroded into the terrain, creating V-shaped indentations in the contour lines? Spot heights are also not just for land; see how the surface water of Lough Fad is recorded at 125m.

Note here, how the contour lines along the coast, north of Pollet Head are almost inseparable, indicating a very high, steep terrain, resulting perhaps in vertical cliffs. Remember also that while several contours can merge into one line, they can never cross over each other.

Interpretation of contours is helped by the use of shading. See how the two examples given above use shades of green and brown to give an impression of different height.

Contours on all OSi mapping are generated from high-flown aerial photography. Flown at 20,000 feet, the photography is used to generate a Digital Terrain Model (DTM) with a regular grid of spot heights at 10m intervals. From this DTM, we interpolate the contour lines that you see on our maps. Once produced, they are subject to a systematic testing process, to ensure accuracy.