Which Trail Would You Take?
You’re out mountain climbing with your friends. You pull out your map and see two hiking trails.
The first route has closely-spaced contour lines, while the second route has widely-spaced spaced contour lines. Which hiking trail would you take?
For the slow-and-steady climber, you’d pick the second route because widely-spaced contour lines mean the slope is gradual. But if you picked the first route, you’re the type of free climber who scales mountains with just a pick axe. This is because when contour lines are close together, it means that slope is steeper.
So how do you read contour lines? Let’s take a nose-dive into some examples.
Depression or Mountain Contour Lines
When you have a closed contour like the one below, this means there is a hill/mountain or depression.
You don’t really know unless there is a label on the contour line. In the example below, we’ve added labels and it should be clear that it’s a depression.
…And this isn’t just any depression. This is the massive meteor crater that struck Arizona.
On some maps, cartographers use teeth marks or hachures for depressions at craters or volcanoes because it marks the elevation goes up or down.
Mount Fuji’s Contour Lines
In this case, closed contour lines indicate a mountain. The beautiful Mount Fuji stands 3,776 meters tall above sea level. At 250-meter spacing, here’s how the contour lines look:
When you see Mount Fuji in 3D, you can see that it’s quite steep as the contours are relatively closely spaced together.
So far, we’ve seen examples of two types of closed contours, but how do contours look like in valleys?
The Rule of V and U-Shaped Valleys
Valleys are elongated low-lying depressions usually with a river flowing through it. You know that you’re looking at a valley bottom when contour lines are V or U-shaped.
Here is an example of a 3D view of the picturesque Collayomi Valley in California:
All rivers flow downhill from higher to the lower elevations, perpendicular to the contour line above it. As a rule of thumb, the V-shaped contour is pointing upstream (the opposite direction from the flow of a stream or river).
The “V” shape contours indicate streams and drainage. As you can see, the “V” points uphill to a higher elevation. Generally, you can connect the apexes of the upward-pointing, “V” shaped contour lines to delineate a stream. Further to this, you can estimate a stream gradient by counting the number of contours that cross a stream with the contour interval.
Slope is always perpendicular to contour lines. The less separation between contour lines means steeper slope and vice versa. When there are evenly spaced contours, slope is uniform.
At a stream junction, contour lines form a “M” or “W” shape. This can be interpreted as two “V” shaped contours intersecting.
What About Ridges and Gullies?
Over time, gullies form through erosion of running water on hillsides. A consequence of two eroded gullies is a spur at the center on the face of a hillside that sticks out. Both gullies and spurs run from ridge lines to valley bottoms.
Gullies (or draws) are characterized by “U” or “V” shaped contour lines with their closed end pointing towards higher elevation. On the other hand, spur contour lines point toward lower elevation.
In 2D, these valley landscape features are a bit more difficult to see. But it’s key to remember how ridges point downslope and gullies point upslope.
Why do Contour Lines Never Cross?
It’s unlikely that contour lines cross, but sometimes they do. When the terrain is an overhang or cliff, contour lines will cross or touch.
The cliffs in Látrabjarg, Iceland are up to 440 meters tall. When you generate 100-meter contours, they are very close to converging.
In 3D, you can see how steep these cliffs are:
Using your imagination, picture an overhang where the terrain hangs outward. This would be a rare exception when two contour lines that cross.
How to Draw Contour Lines
You can easily generate contour lines with the click of a button in CAD or GIS software. But what if you want to draw it by-hand?
The first thing you have to do is choose a contour interval. In our example, we’re going to use a contour interval of 10 meters.
Between transition points of ten (30, 40, 50, etc), add markers where to draw lines.
Now that we have points to connect, let’s draw smooth contour lines for each interval.
And there we have it.
The Purpose of Contour Lines
In our example, contour lines represented constant elevation and showed the topography of the landscape.
But, contour lines are used for meteorology (isopleth), magnetism (isogon) and even drive-time (isochrones).
The closeness of contour lines indicate slope. Irregular contours mean rugged terrain. It’s rare in nature for contours to cross.
What are some other types of contours you’ve seen in nature?