A contour interval is the vertical distance between adjacent contour lines on a topographic map. If a map has a contour interval of 20 feet, each line you cross represents a 20-foot change in elevation. It’s the key number that lets you read a flat, two-dimensional map and understand the three-dimensional shape of the land.
How Contour Lines Work
A contour line connects all points on a map that sit at the same elevation. Picture a mountain with water rising around it in perfectly even steps. Each waterline would trace a contour. On a topographic map, these lines do the same thing: they show you where the land is high, where it’s low, and how steeply it changes in between.
The contour interval is the fixed vertical gap between each of those lines. On a map with a 40-foot contour interval, the first line might represent 5,200 feet of elevation, the next 5,240 feet, the next 5,280 feet, and so on. The interval stays consistent across the entire map so you can compare terrain at a glance.
Three Types of Contour Lines
Not every contour line looks the same. Topographic maps use three distinct types:
- Index contours are the thicker, bolder lines that appear every fourth or fifth contour. They’re labeled with their elevation, making them your anchor points for reading the map.
- Intermediate contours are the thinner, unlabeled lines between index contours. They represent each standard interval step but keep the map from looking cluttered with numbers.
- Supplementary contours appear as dotted or dashed lines in areas where the terrain is so flat that the regular interval doesn’t capture meaningful changes. On a standard USGS map, these often represent 5-foot intervals in otherwise featureless terrain.
How to Calculate a Contour Interval
Every topographic map prints its contour interval somewhere in the margin, but if that information is missing or you want to verify it, the math is straightforward. Find two adjacent index contours (the bold, labeled lines) and subtract the lower elevation from the higher one. Then count the number of unlabeled contour lines between them and add one, because those lines divide the space into one more section than there are lines.
For example, if two index contours are labeled 5,200 feet and 5,400 feet, the elevation difference is 200 feet. If there are four intermediate lines between them, those four lines divide the 200 feet into five equal sections. Divide 200 by 5, and you get a contour interval of 40 feet. You can check your work by starting at the 5,200-foot index contour and counting up by 40 feet each time you cross a line. You should land exactly on 5,400 feet.
Why the Interval Changes Between Maps
Cartographers choose a contour interval based on the terrain the map covers. Standard USGS topographic maps at the common 1:24,000 scale typically use a 20-foot interval, but that’s not universal. Intervals of 5, 10, and 40 feet all appear on maps of the same scale depending on the landscape.
In flat areas like coastal plains or river valleys, a 20-foot interval might mean contour lines barely show up at all, hiding subtle but important changes in the land. A smaller interval of 5 or 10 feet captures those details. In mountainous terrain, a 5-foot interval would pack lines so tightly together they’d be unreadable. A 40-foot interval keeps the map clean while still showing the shape of ridges and valleys. The goal is always the same: choose an interval that makes the terrain legible without overwhelming the reader or losing important features.
Reading Slope From Line Spacing
Once you know the contour interval, the horizontal spacing between lines tells you how steep the ground is. Lines packed close together mean the elevation is changing rapidly over a short horizontal distance, which is a steep slope. Lines spread far apart mean the same elevation change is happening gradually, indicating gentle terrain.
This is one of the most practical things you can do with a contour interval. If you’re planning a hike and see contour lines bunched tightly on one side of a ridge but spread wide on the other, you know one approach is significantly steeper. The contour interval gives you the vertical dimension, and the spacing gives you the steepness. Together, they turn a flat sheet of paper into a realistic picture of what the ground actually looks like.
Contour Intervals in Digital Maps
Modern digital elevation data doesn’t rely on drawn contour lines the way paper maps do, but the concept of vertical resolution plays the same role. Digital elevation models store the height of the ground at regular grid points, and the spacing and accuracy of those points determine how much terrain detail you can extract. The most common global datasets use a 30-meter grid resolution, meaning one elevation value for every 30-meter square of ground.
When software generates contour lines from this data, you can set any interval you want. But the accuracy of those contours still depends on how precise the underlying elevation measurements are. Industry standards from organizations like the American Society for Photogrammetry and Remote Sensing treat vertical accuracy as the primary measure of elevation data quality. A dataset with poor vertical accuracy will produce contour lines that look clean but misrepresent the actual terrain, so the source data matters as much as the interval you choose to display.