Contour index, fixing contours between 1240.12 and 1264.32

Contour index

An index contour is every fifth contour, drawn as a thicker line and labelled with its RL so that contours can be identified easily. With CI = 3 m, the index contours are at every 15 m: 1245 m, 1260 m, ... The contours between them are the intermediate (minor) contours.

Fixing index and minor contours between 1240.12 m and 1264.32 m

The ground between the two points has a uniform slope, so contour positions are found by proportion.

1. Contour values are multiples of CI = 3 m. Between 1240.12 and 1264.32 these are 1242, 1245, 1248, 1251, 1254, 1257, 1260, 1263 m.
2. Of these, 1245 m and 1260 m are multiples of 15 m, so they are index contours; the other six are minor contours.
3. The difference in level between the points is $1264.32 - 1240.12 = 24.20$ m. If the points are $L$ cm apart on the map, the distance of a contour of value $h_c$ from the lower point is

$$x = \frac{h_c - 1240.12}{24.20}\times L$$

Taking the plotted distance as $L = 12.1$ cm (so that $x = (h_c - 1240.12)/2$ cm):

4. Mark these positions on the line joining the points, repeat for the neighbouring points, join equal values with smooth curves, and draw the 1245 m and 1260 m contours thick and label them.

 1240.12  1242 1245 1248 1251 1254 1257 1260 1263   1264.32
   o-------|----||---|----|----|----|----||---|-------o
                 idx                     idx

The spacing between neighbouring contours is equal ($3/24.2\times 12.1 = 1.50$ cm) because the slope is uniform.

Why contour lines never touch or cross

All points of a contour have the same RL. If two contours of different RL touched or crossed, the common point would have two different elevations, which is impossible. The only exceptions are a vertical cliff, where contours of different value fall on the same plan line and appear to merge, and an overhanging cliff or cave, where the upper contour is drawn over the lower one and appears to cross it (the hidden part is dotted).