What Is A Contour Interval And How Do You Read It?

A contour interval is the vertical distance between two adjacent contour lines on a topographical map. It tells you how much elevation changes between each line, allowing you to visualize the steepness and shape of the terrain. To read it, you find the elevation difference between two labeled lines and divide by the number of spaces between them, giving you a fixed number that applies across the entire map.

What Is A Contour Interval And How Do You Read It?

A contour interval is a fixed number printed in the map legend. It represents the constant vertical rise or drop between each successive contour line. For example, if a map has a contour interval of 20 feet, every line you cross moving uphill represents a gain of exactly 20 feet in elevation.

Reading it requires two pieces of information: the interval itself (found in the legend) and the index contours. Index contours are thicker, darker lines that appear every fourth or fifth line and have their elevation printed directly on them. By counting the thinner lines between two index contours, you can confirm the interval and use it to calculate the elevation of any point on the map.

Why Does the Contour Interval Change Between Maps?

Mapmakers do not use one universal contour interval. The interval is chosen based on the terrain being mapped and the purpose of the map. A flat coastal region might use a 5-foot interval, while a mountainous national park might use a 100-foot interval. If the interval were too small in mountainous terrain, the lines would merge into a solid blob of ink.

The interval also depends on map scale. A large-scale map (showing a smaller area in detail) can afford a smaller interval. A small-scale map covering hundreds of miles must use a larger interval to keep the map readable. You should always check the legend before interpreting any map, because the interval is not consistent across different maps of the same region.

How Do You Calculate Elevation Using the Contour Interval?

Find the two index contours that bracket your location. Read the printed elevations on both. Subtract the lower from the higher, then count the number of spaces between them. Divide the elevation difference by the number of spaces.

Here is a concrete example using a standard 100-foot interval map:

• Index contour A reads 1,000 feet
• Index contour B reads 1,500 feet
• There are 5 spaces between them
• 500 divided by 5 equals a 100-foot contour interval

Once you know the interval, you can determine the elevation of any unlabeled line. If you are standing on the second thin line above the 1,000-foot index, your elevation is 1,200 feet. This works in both directions — moving downhill subtracts the interval for each line crossed.

What Do Contour Lines Tell You About Terrain Shape?

Contour lines do more than show elevation. The spacing between them reveals the slope. Lines that are close together indicate a steep slope because the elevation changes rapidly over a short horizontal distance. Lines that are far apart indicate a gentle slope. Evenly spaced lines suggest a uniform slope, while lines that gradually spread out suggest a slope that flattens toward the bottom.

Closed circles on a map represent a hilltop or a depression. If the closed circle has hachure marks (short lines pointing inward), it is a depression, not a hill. When contour lines form a V shape, they indicate a valley or ridge. The V points uphill in a valley and downhill on a ridge — a distinction that experienced map readers use constantly.

Why Does the Contour Interval Matter for Navigation?

For hikers, hunters, and land surveyors, the contour interval directly affects route planning. A map with a 10-foot interval shows subtle terrain features that a 50-foot interval would hide. Small benches, shallow drainages, and minor ridges disappear when the interval is too large. Conversely, a very small interval on rugged terrain creates a dense, cluttered map that is hard to read in the field.

Understanding the interval also helps you estimate physical effort. If you know a trail climbs 400 feet per mile and you are carrying a heavy pack, you can plan rest stops accordingly. Without knowing the interval, you cannot tell whether a map is showing a gentle rolling hill or a serious climb. The interval is the key that translates map lines into physical reality.

For safety, always check the contour interval before relying on a map for navigation. A map that looks flat at a 100-foot interval may actually contain dangerous cliffs that would be obvious on a map with a 10-foot interval. The same terrain can look completely different depending on the interval used.

How Are Contour Intervals Determined for a Specific Map?

Surveyors and cartographers select the interval based on the map’s intended use and the terrain’s relief. Relief is the difference between the highest and lowest points on the map. A map with high relief over a small area typically needs a larger interval to keep lines from overlapping. A map with low relief can use a smaller interval to show subtle changes.

There are established guidelines for interval selection, but they are not universal laws. In general, the interval should be small enough to show important terrain features but large enough to keep the map legible. For specialized maps — such as those used for floodplain analysis or engineering projects — intervals may be much smaller than what a recreational hiker would use.

Some digital mapping tools allow you to adjust the contour interval on demand. This is useful but has a limitation. The underlying elevation data has a fixed resolution. If you zoom in and request a contour interval smaller than the data supports, the resulting lines will be inaccurate or misleading. The map can only show detail that the source data actually contains.

Common Mistakes When Reading Contour Intervals

The most common mistake is assuming all contour lines on a map have the same interval. They do — within a single map, the interval is constant. But people often forget to check the legend and assume a familiar interval from a previous map applies. This can lead to significant elevation errors, especially when switching between maps of different scales.

Another frequent error is miscounting the spaces between index contours. Remember that you count the spaces, not the lines. If you count the lines themselves, you will be off by one interval. This might not matter on flat terrain, but it can be a serious error in mountainous areas where each interval represents 100 feet or more.

A third mistake is confusing contour intervals with map scale. Scale describes horizontal distance (such as 1 inch equals 1 mile). The contour interval describes vertical distance. They are independent. A map can have a large scale and a large contour interval, or a small scale and a small contour interval. Do not assume one tells you anything about the other.