Contour Interval Calculator
Free Contour interval Calculator for geomorphology & mapping. Enter variables to compute results with formulas and detailed steps.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
CI = Elevation Range / Desired Contours (rounded to standard interval)
The raw interval is the elevation range divided by the desired number of contours, then rounded to the nearest standard cartographic interval (1, 2, 5, 10, 20, 50, 100, 200, 500 m).
Worked Examples
Example 1: Mountain Region Topographic Map
Problem:Select a contour interval for a 1:50000 map covering terrain from 500 m to 2500 m elevation aiming for approximately 20 contour lines.
Solution:Elevation range = 2500 - 500 = 2000 m\nRaw interval = 2000/20 = 100 m\nNearest standard = 100 m\nNumber of contours = 2000/100 = 20\nIndex interval = 500 m\nFirst contour = 500 m, Last = 2500 m
Result:Interval: 100 m | 20 contours | Index every 500 m
Example 2: Coastal Plain Mapping
Problem:Map terrain from 5 m to 85 m elevation at 1:25000 scale with about 20 desired contours.
Solution:Elevation range = 80 m\nRaw interval = 80/20 = 4 m\nNearest standard = 5 m\nNumber of contours = 80/5 = 16\nIndex interval = 25 m\nFirst contour = 5 m, Last = 85 m
Result:Interval: 5 m | 16 contours | Index every 25 m
Frequently Asked Questions
What is a contour interval and why does it matter?
A contour interval is the constant vertical distance between adjacent contour lines on a topographic map. Choosing the right interval is critical because too small an interval creates cluttered unreadable maps while too large an interval obscures important terrain features. Standard intervals follow a progression of 1, 2, 5, 10, 20, 50, 100, 200 and 500 meters depending on the elevation range and map scale. The interval determines how much topographic detail the map can convey and directly affects the density of lines in steep versus gentle terrain. Military topographic maps typically use a fixed interval for each scale while thematic maps may adjust intervals for specific purposes.
How is contour interval related to map scale?
Map scale and contour interval are closely linked because the physical spacing between contour lines on the printed map must remain legible. At large scales like 1:10000 small contour intervals of 1 to 5 meters are appropriate because the ground distance represented by each centimeter is small. At small scales like 1:250000 intervals of 50 to 100 meters are needed to prevent excessive line crowding. A general rule of thumb sets the contour interval in meters to approximately one-thousandth of the scale denominator so a 1:50000 map uses a 50-meter interval. This ensures contour lines are spaced far enough apart to be distinguishable even on moderate slopes.
How do you determine the first and last contour lines?
The first contour line on a map is the lowest elevation that is a whole multiple of the contour interval at or above the minimum elevation in the mapped area. For example if the minimum elevation is 487 meters and the contour interval is 20 meters the first contour is 500 meters. Similarly the last contour is the highest whole multiple of the interval at or below the maximum elevation. If maximum elevation is 2543 meters with a 20-meter interval the last contour is 2540 meters. The actual summit and valley bottom elevations are indicated by spot heights rather than contour lines. This systematic approach ensures all contour lines across the map represent elevations that are exact multiples of the chosen interval.
How does terrain steepness affect contour line spacing?
Contour line spacing on a map is inversely proportional to terrain steepness. On steep slopes contour lines are packed closely together because the elevation changes rapidly over a short horizontal distance. On gentle slopes contours are widely spaced reflecting gradual elevation change. A vertical cliff would show contour lines merging into a single thick line while a perfectly flat plain would have no contour lines at all. This visual relationship between spacing and steepness is one of the most powerful features of contour maps allowing trained readers to instantly perceive the three-dimensional shape of terrain from a two-dimensional representation.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy