How to Calculate Index Contour Interval

Index contour interval is a fundamental concept in cartography and geographic information systems (GIS). It represents the vertical distance between contour lines on a map, which helps visualize terrain elevation changes. This guide explains how to calculate index contour interval, its importance, and provides an interactive calculator to perform the calculation.

What is Index Contour Interval?

In cartography, contour lines connect points of equal elevation on a topographic map. The index contour interval is the vertical distance between these lines, typically marked with thicker lines and labeled with elevation values. It serves as a reference for understanding terrain features and elevation changes.

Index contour intervals are chosen based on the map's scale and the terrain's complexity. A smaller interval provides more detailed elevation information, while a larger interval offers a broader overview. The choice of interval affects how accurately the map represents the terrain.

Index Contour Interval Formula

The index contour interval is determined by the map's scale and the desired level of detail. The general formula for calculating the index contour interval is:

Index Contour Interval = (Map Scale × Vertical Exaggeration) / 1000

Where:

Map Scale - The representative fraction of the map (e.g., 1:25,000)
Vertical Exaggeration - A factor that enhances vertical features (typically 1 for standard maps)

For most practical purposes, the vertical exaggeration is set to 1, simplifying the calculation to:

Index Contour Interval = Map Scale / 1000

How to Calculate Index Contour Interval

To calculate the index contour interval, follow these steps:

Determine the map scale. This is typically provided on the map legend or documentation.
If the map has vertical exaggeration, note this value. For standard maps, use 1.
Divide the map scale by 1000 to get the index contour interval.
Round the result to a practical value, usually to the nearest 5 or 10 units.

For example, if the map scale is 1:25,000, the index contour interval would be 25 meters (25,000 ÷ 1000).

Note: The actual interval used on a map may be adjusted based on the terrain's complexity and the map's purpose. The formula provides a starting point for determining an appropriate interval.

Worked Example

Let's calculate the index contour interval for a map with a scale of 1:50,000 and no vertical exaggeration.

Map Scale = 1:50,000
Vertical Exaggeration = 1
Index Contour Interval = 50,000 ÷ 1000 = 50 meters

The calculated index contour interval is 50 meters. In practice, the map might use 50 meters or adjust to 40 or 60 meters based on the terrain's features.

Example Calculation
Map Scale	Vertical Exaggeration	Calculated Interval	Practical Interval
1:25,000	1	25 meters	20 meters
1:50,000	1	50 meters	50 meters
1:100,000	1	100 meters	100 meters

FAQ

What is the difference between contour interval and index contour interval?

The contour interval refers to the vertical distance between all contour lines, while the index contour interval is the interval between the thicker, labeled contour lines. Index contours are typically every 5th or 10th contour line, depending on the map's scale and purpose.

How do I choose the right index contour interval for my map?

The index contour interval should be chosen based on the map's scale and the terrain's complexity. A smaller interval provides more detail, while a larger interval offers a broader overview. The formula provided in this guide can help determine an appropriate starting point.

Can I use the same interval for all contour lines?

No, using the same interval for all contour lines can make the map less readable. Index contour intervals are typically thicker and labeled to provide reference points, while other contour lines use a smaller interval for more detailed elevation information.