Difference between Index, Intermediate and Supplementary Contour lines

Sometimes a map may have several contour lines such that it becomes difficult for the cartographer to label the elevations of each contour line. Thus, in order to make the map easier to read and less cumbersome, every fifth (5th) contour line from the mean sea level is usually labelled with its elevation and sometimes bolded a little bit heavier than the rest. These bolded and labeled contour lines are usually referred to as the index contour lines.

The remaining contours are called intermediate contours. These usually fall in between the index contour lines. The Intermediate contour lines are finer than index contours and do not have their elevations labelled on them. There are normally four intermediate contour lines between any two successive index contour lines.
Recommended to read: contour lines on topographic maps

In some certain areas on a map, the normal contour interval is sometimes too large to represent a significant change in elevation, such as on level terrain, and thus, additional half-interval contours are added. These kinds of contours are called supplementary contours or form lines.
They usually appear on the map as dashed or thin continuous lines with no representative value of elevation on them. They are used when the overall change in elevation is very gradual.
When supplementary contours appear on the map, the contour interval used for them is usually specified in the contour interval note.

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