Contour lines: what they are and how to read them
Contour lines turn relief into lines on flat paper. With a little practice you can read the shape of the ground in them: where it rises, by how much, where water runs down and where it is easy to pass.
What a contour line is
A contour line joins every point of the ground that is at the same elevation. Picture a mountain sliced horizontally, each slice the same distance above the last: the edge of each slice seen from above is a contour line.
That is why two contours never cross (a point cannot be at two heights at once, except under an overhang) and each contour eventually closes on itself, though sometimes off the paper.
Walking along a contour means walking on the level: neither up nor down. Crossing them means climbing or descending.
The contour interval and index contours
The contour interval is the difference in elevation between two consecutive contours. On a map with a 20 m interval, crossing five contours means climbing or descending 100 m.
Every so many contours, one is drawn thicker and labelled with its elevation: the index contour. The usual pattern is one index contour every five, so with a 20 m interval they mark 100, 200, 300 m... They save you counting contours one by one.
The interval depends on the scale: if it were always the same, a detailed map would have contours too far apart and a regional one a tangle. PlotMyMap adjusts it so that two contours sit a little under a millimetre apart on paper on an ordinary slope: at 1:10,000 they come every 10 m, at 1:25,000 every 20 m and at 1:50,000 every 50 m. Index contours are one in five, with the height written along the line itself.
Steepness: close and spread contours
The basic rule: close contours, steep slope; spread contours, gentle ground. Where contours almost touch there is a cliff or a very steep slope; where they open wide, a plain, a plateau or a valley floor.
You can work out the slope. Measure the distance between two contours on the map, convert it to metres with the scale and divide the interval by that distance. At 1:25,000 with 20 m contours, if two contours are 2 mm apart the real distance is 50 m: the slope is 20 / 50 = 40 %. If they are 8 mm apart (200 m), it is 10 %.
For reference: up to 10 % is comfortable walking, 20 to 30 % is a serious climb, and from 50 % you need your hands or a zigzag path.
Valleys, ridges, summits and saddles
Summits: closed, concentric contours getting smaller towards the centre. The smallest is the top; topographic maps usually mark it with a dot or triangle and its elevation.
Valleys and gullies: contours form a V or U whose tip points uphill. A stream often runs along the bottom of that V, and water flows away from the tip.
Ridges and spurs: they also form a V or U, but with the tip pointing downhill, towards the valley. A handy way to tell them apart is to look at the index contour heights: if the tip points towards lower heights, it is a spur; if towards higher heights, a valley.
Saddles: the low point between two summits, shaped like a riding saddle. On the map it shows as two facing groups of closed contours with a narrowing between them. They are natural passes and paths usually cross through them.
Hollows and sinkholes: closed contours enclosing lower ground. Many maps mark them with small ticks pointing inwards; in limestone country, like the Picos de Europa, they are common.
Reading the map as a landscape
With the basic shapes you can picture the ground before going. Look first for the rivers (always at the bottom of the Vs), then the summits (the highest closed contours), then the ridges that join them. With those three elements the relief falls into place.
Look at how paths run relative to the contours: a path running parallel to them is level; one crossing them at right angles, steep; a zigzag, a hard climb that the path eases.
A good exercise is to compare the map with a photo, or with the landscape itself on a hike: you quickly learn to “see” the mountain in the lines.
Where contours come from
The contours of a modern map are computed from a digital elevation model: a grid with the elevation measured at each point, usually by laser from an aircraft (LIDAR) or by radar from a satellite.
In Spain, PlotMyMap uses the model of the Instituto Geográfico Nacional, with a sample every 5 m on detailed maps. Elsewhere it uses SRTM, NASA's radar mission, with a sample every 30 m. Contours are smoothed just enough to avoid stair-steps, without inventing relief that is not in the data.
No model is perfect: on vertical cliffs, in very dense forest or in towns there may be small errors. So a home-made map is great for planning a route or hanging on the wall, but it does not replace official maps or a compass for mountain navigation.
Making your own contour map
In PlotMyMap, the topographic map type comes with contour lines including index contours and heights, peaks with their elevation, UTM grid, scale bar, north arrow and legend.
Choose the scale for what you want to read: 1:10,000 for a hillside or a mountain village in full detail, 1:25,000 for a day route, and 1:50,000 and beyond for a whole range. From 1:30,000 the map changes register and the contours become the structure of the drawing.
The Potes (1:10,000) and Picos de Europa (1:25,000) example maps are a good starting point to see how reading changes with scale.
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