A contour line is a simple promise: every point along it sits at the same height above sea level. Everything else in map reading grows out of that one idea. Once we can see a hillside in a bundle of brown lines, the map stops being a drawing and becomes a model of the ground we are about to walk. This article explains the four shapes that matter most on a hike, and why the spacing between the lines, the contour interval, quietly decides how much the map can tell us.
What the lines actually say
Contours are drawn at fixed vertical steps. The horizontal distance between two adjacent lines therefore tells us the gradient: lines packed tightly together mean steep ground, lines spread apart mean gentle ground, and a wide blank area is close to flat. Because every line is a closed loop somewhere on the planet, contours never cross, never split and never simply end.
Two conventions make the reading faster. Every fourth or fifth line is usually printed thicker and labelled with its height; these index contours give us the scale of a slope at a glance. And the numbers on index contours are traditionally printed so that the top of the figure points uphill, which is often the quickest way to tell a hill from a hollow when the shapes look identical.
Spurs and re-entrants: the U that points two ways
The most common contour shape in any hilly terrain is a U or V. The trick is knowing which way it points. A spur is a ridge of ground that sticks out from a hillside, like a knuckle on a fist. Its contours form a U whose closed end, the rounded bottom of the letter, points downhill, towards lower ground. A re-entrant is the opposite: a small valley or gully cut back into the slope, and its contours form a U whose closed end points uphill, towards higher ground.
The reliable test is to find the height labels or the nearest index contours and ask which side is higher. If the tip of the U reaches towards the lower numbers, we are looking at a spur. If it reaches towards the higher numbers, it is a re-entrant. Streams almost always run down re-entrants, so a thin blue line inside the U settles the question instantly. Spurs, in contrast, are usually dry and often carry paths, because they offer gentler and better drained walking than the gullies either side.
The distinction matters because spurs and re-entrants alternate across every hillside. A route that follows a spur keeps us on a broad, navigable line; a route that mistakes a re-entrant for a spur walks us into a steepening trough that may end in a waterfall or a boulder choke.
Saddles, cols and the hourglass
A saddle, also called a col or a pass, is the low point on a ridge between two summits. On the map it appears as an hourglass: two sets of closed loops (the summits) pinched together, with the contours on either side of the pinch opening out downhill into the valleys. Stand on a saddle and the ground rises in two opposite directions and falls in the other two. It is the natural crossing point of a ridge and the place where paths converge, which also makes it the classic spot to take a bearing and to reassess conditions before committing to the far side.
Saddles are also where wind accelerates as air squeezes through the gap; a col that reads as a gentle notch on paper can be the most exposed point of the day.
Knolls, cliffs and the shapes that do not fit
A small closed loop with nothing inside is a knoll, a bump of ground that may be only a few metres high but is often enough to hide the next stretch of path. A hollow with no outlet, common in limestone country, is shown with the same closed loop but with short tick marks pointing inward, called hachures. Where contours come so close that they merge, most mapping agencies switch to a cliff symbol; the message is not merely “steep” but “do not plan a route through here”.
Convex and concave slopes deserve a word too. A convex slope has contours spaced widely at the top and tightly at the bottom, which means that from above we cannot see the steep part until we are on it. A concave slope is the reverse and can be read from the summit. Descending in poor visibility, we treat every convex profile on the map as a hazard.
Why the interval changes everything
Here is the part that catches experienced walkers. The contour interval is the vertical distance between adjacent lines, and it is set by the map’s scale and its publisher. Common intervals are 5 m, 10 m and 20 m (roughly 16, 33 and 66 ft); some mountain maps use 25 m or 40 m. The interval is printed in the legend, and it decides what the map can and cannot show.
Consider a rock step 15 m (50 ft) high. On a 10 m map it will appear as one or two contours suddenly bunched together, an obvious warning. On a 20 m map the same step may fall entirely between two lines and vanish. A 20 m interval can hide an entire knoll, a dry gully deep enough to lose a person in, or a terrace that would make an easy traverse. The ground has not changed; our knowledge of it has been rounded down.
The interval also changes how gradient reads. On a 1:25,000 map with 10 m contours, 1 mm between lines works out at 25 m horizontal for 10 m vertical, a slope of about 22 degrees. On a 1:50,000 map with 20 m contours, the same 1 mm gap represents 50 m horizontal for 20 m vertical, which is the same 22 degrees but with half the detail. Lines that look comfortably spaced on the coarser map can still contain slopes that would be alarming on the finer one. Before every trip we check the interval, and when we switch between two maps of the same area we recalibrate our eye deliberately.
Turning shapes into a route
Good contour reading is a habit of comparing the model to the terrain. A few checks pay off every time:
- Read the legend first: scale, contour interval and which lines are index contours.
- Find the high ground before naming any U shape; height decides spur versus re-entrant.
- Trace the route with a fingertip and count the contours crossed in each leg to estimate ascent.
- Mark saddles and convex slopes as decision points, not just waypoints.
- On the ground, look for the shape you expect before you look at the map, then confirm.
Counting contours turns the map into a timing tool. If a leg crosses fifteen 10 m contours, that is 150 m (about 500 ft) of climb, roughly fifteen extra minutes under a standard rule of thumb. The brown tangle becomes an itinerary with numbers attached, and the numbers are what keep a good walk from turning into a long one.