Counting each contour line up to and including the next index line.
To find the elevation interval between the contour lines,
Divide the difference in elevation between the index lines by the number of contour lines from one index line to the next.
In the example above, the distance 200 is divided by the number of lines, 5.
The contour interval is equal to 200 / 5 = 40,
or 40-unit contour intervals.
If, on the other hand, the elevation difference between the index lines were 100 feet, the contour interval would be 100 / 5 = 20 or a 20-unit contour interval.
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The Scale of The Map: The contour interval is inversely proportional to the scale of the map. If the scale is small, the contour interval should be large and vice versa.
Nature of The Ground: The general terrain of the area determines the contour interval. For a flat area, the contour interval should be small. For sloping or undulating terrain, the chosen contour interval should be large
Purpose and Expansion of the survey work: If surveying work is to be used for precise and detailed calculations, then the small contour interval should be chosen and a large contour interval is chosen for the following cases: For catchment areas, To the reservoir, For location survey.
Time available and Eligible Expenses for field and office work: A large contour interval should be used if the time available for survey work is less. The small contour interval, survey work, and plotting need to be more precise. For a small contour interval, the money needed will also be more as the field and office work will be larger.
Index lines are the thickest contour lines and are usually labeled with a number at one point along the line. This tells you the elevation above sea level.
Intermediate lines are the thinner, more common, lines between the index lines. They usually don't have a number label. Typically one index line occurs for every five intermediate lines.
Supplementary lines appear as dotted lines, indicating flatter terrain.
How Do You Read Contour Intervals on a Topographic Map?
Index lines are the thickest contour lines and are usually labeled with a number at one point along the line. This tells you the elevation above sea level.
Intermediate lines are the thinner, more common, lines between the index lines. They usually don't have a number label. Typically one index line occurs for every five intermediate lines.
Supplementary lines appear as dotted lines, indicating flatter terrain. If you’re looking at an index line, it's easy to read the elevation because it is clearly labeled. However, interval lines are somewhat trickier. To determine their elevation, you’ll need to know the contour intervals.