Contour interval, index contour, why never cross

Contour interval

Contour interval (CI) is the constant vertical distance between two consecutive contours, for example 1 m or 5 m. It is the same over one map. It depends on the scale, relief of the ground and the purpose of the map.

Contour index

Index contour is every fifth contour (for example every 5th line when CI = 2 m, giving 10 m, 20 m, 30 m ...) drawn as a thicker line and labelled with its RL, so that the map can be read quickly. The other contours are called intermediate (minor) contours.

Plotting index and minor contours

The contours are drawn from the spot levels of surveyed points by interpolation. Example: two surveyed points A (RL 52.4 m) and B (RL 61.8 m) plotted 9.4 cm apart; CI = 2 m, so the index contour is every 5th contour (multiples of 10 m).

1. List the contour values between the two RLs: 54, 56, 58, 60 m. Of these, 60 m is an index contour (multiple of 10 m); 54, 56 and 58 are minor contours.
2. Assuming a uniform slope, find the distance of each contour from A: $x = \dfrac{h_c - 52.4}{61.8 - 52.4}\times 9.4$.

3. Mark these points on AB, repeat for all the pairs of points, and join the points of equal value with smooth curves.
4. Draw the index contour (60 m) thicker, with its value written in a break in the line; the minor ones are thin lines.

   A(52.4)  54   56   58  60(index)  B(61.8)
   o--------|----|----|----||--------o

Why contours never touch or cross

A contour joins points of one fixed elevation. If two contours of different elevations touched or crossed, the common point would have two different elevations at the same place, which is impossible. So contour lines never meet. The only exceptions are:

• a vertical cliff, where the contours of different RL fall on the same plan position and appear to merge into one line, and
• an overhanging cliff or cave, where the upper contour projects over the lower, so the lines appear to cross (the hidden part is drawn dotted).