This video is the best explanation you'll find on YouTube about how contour lines show elevation on topographic maps. Learn definitions and examples of contour lines, index contours, and contour interval, and try your hand at determining elevations and calculating contour interval.
This video is part of a series on wilderness navigation and orienteering. See the complete series on our YouTube channel, or at our website, www.croc.org
When looking at elevation and contours, there are three key terms we need understand.
The contour line, the contour interval, and index contour.
Let's start with the contour line.
Imagine you’re on a small island in the ocean.
The high point of your island is about 35 feet, and it's a few hundred feet around at the water’s edge.
Let's also imagine that you have a really big piece of chalk.
Now, imagine yourself walking from sea level up to an elevation of 10 feet.
You now walk completely around the island at 10 feet of elevation, drawing a mark every few steps with the chalk. You walk more or less in a circle, and arrive back at your exact starting position in a few minutes. You changed direction, walking north, east, south, and west all around the island, but you stayed on the same 10 foot elevation line the entire time.
Now, you walk uphill 10 more feet to the 20 foot elevation, and repeat the previous process.
You again walk around the island at the 20 foot contour line, leaving a trail of chalk marks.
You repeat this one more time, walking uphill 10 more feet to the 30 foot line, and walking around and marking what little remains of the island at elevation 30 feet.
Now, you've drawn three lines. Each loop is 10 feet higher than the preceding one, and all the loops are getting smaller as you get towards the top of the island.
A bird’s eye view of your island is on the right, which is a perfect contour map of your island.
So, this gives us a definition of a contour line - a continuous line of constant elevation.
The second of our three terms is a contour interval.
An interval is the elevation difference between each contour line.
In our example with the island, the interval was 10 feet.
Intervals can be in meters or feet, and can be different on every map. On the left is a small scale map from Canada, with an interval of 50 meters. On the right is a 7 1/2 minute map from the US Geological Survey, with interval of 40 feet.
Sometimes the interval is printed right on the map, and sometimes you have to figure it out yourself.
Contour intervals are almost always multiples of 5, such as 5, 10, 20, 40 or 80.
The interval chosen by the mapmaker depends on how steep the terrain is.
Intervals may range from 5 feet in very flat areas, like Florida, to 150 or more feet very steep terrain or in small-scale maps.
Usually in mountainous areas, the interval is 40 feet.
Now, let's have a look at our third key term, the index contour.
Here we see a map of the Emmons Glacier on Mt. Rainier.
A little quick math shows us that these lines are 200 feet apart.
So, knowing this, we can determine the contour interval if it's not printed on the map.
To find the interval, we take the vertical distance between index contours and divide by five.
Here, 200 feet divided by 5 gives us a contour interval of 40 feet.
Now, let's figure out the elevations for each contour line between 4400 and 4600 feet.
First, we need to figure out the contour interval.
The contour interval is 40 feet.
200 feet between the marked index contours, divided by 5.
We add a 40 feet to 4400, and we have the elevation of the next higher contour line.
Again, we add 40 feet on to the previous line, and we get 4480.
Add 40 feet onto the previous line, we get 4520.
And finally, adding 40 feet onto the previous line, we get our last line of 4560.
Once you know what the contour interval is, you can find the elevation of any point on your map. Here's how to do it.
What is the elevation of Camp Schurman?
First, we need to find the contour interval.
200 feet between the index lines divided by 5 gives us in an interval of 40 feet.
What's the contour interval here?
The index lines are 50 feet apart.
Dividing 50 by 5 gives us an interval of 10 feet.
What's the contour interval here?
The index lines are 100 feet apart.
Dividing 100 by 5 gives us an interval of 20 feet.
1 - All of the points on a contour line have the same elevation.
2 - Contour lines never cross each other.
3 - Index contour lines are printed in a bold, usually every fifth line.
4 - Some, but not all, index contours have an elevation printed on them.
5 - Dividing the elevation change between index contours by five gives you the contour interval.
6 - The contour interval is the real world change in elevation between each contour line.
7 - Sometimes this is printed on the map, and sometimes you have to figure it out yourself.
8 - When you know the contour interval, you can determine the elevation of any point on your map.