HOW TO DRAW CROSS-SECTION FROM TOPOGRAPHICAL MAP

A cross-section is a profile of the area under discussion. It is a diagram showing a change in height along a line drawn between 2 points on a map.

In an exam, the vertical scale will always be indicated — you must not change it. e.g. 1 cm represents 50 meters on the Y-axis (vertical axis) Remember: the Horizontal Scale remains 1:50 000 (this will be map scale)

Procedure for drawing cross-sections

BEFORE starting, study the area of the cross-section to the general shape or lie of the land, that is always had an idea of the shape BEFORE you begin plotting heights or drawing the section.

• On a “magic” piece of paper, delimit the cross-section on the map and match it to the “given” graph. Mark off the right-hand extreme on the graph and draw a vertical line to delimit it.
• On the map, place the piece of paper, and (holding it steady) mark off EVERY contour that cuts it, ACCURATELY numbering each mark vertically. E.g. 2 2 2 2 0 2 4 6 0 0 0 0
• Now place the “magic” piece of paper along the baseline of the graph — ensure that the 2 extremes are accurately lined up. Matching the vertical scale to the marked contour height, mark a small “x” to show the position of each contour on the graph.
• YOUR EDGES OF THE GRAPH MUST COINCIDE WITH THE LIMITS OF THE GRAPH — nowhere on earth does a piece of land end in mid-air!!!
• Join these points free hand, taking care to show valleys or hills where 2 or 3 adjacent points are at the same height.
• Write a FULL heading on your cross-section e.g. Cross-section from * 96 to r 281 on 3326 BC Graham's town map extract 1: 50 000 Topographical Series
• Mark in the horizontal scale, Mark in the vertical scale showing the units of measurement Label the edges of the cross-section using the designated points (NOT just A and B

Vertical Exaggeration This is used as the vertical scale must be exaggerated because, if the horizontal scale were used for the vertical, the relief would show as an almost flat line on a cross-section. Formula:

Vertical Exaggeration = Vertical Scale (Given on the cross-section)/ Horizontal Scale (1 cm represent 50000 cm)

Eg Vertical Scale = 1 cm represent 20 metres Convert to cms (an RF) by multiplying by 100 i.e. 1 : 2 000 VE = 1:2000/1:50000 VE=50000/2000/ VE = 25 times

Inter visibility This is the concept of whether one place on a map can be seen from another. It is decided upon by studying the heights between the 2 places. Any ground which cannot be seen behind a higher height is known as DEAD GROUND.

If a convex slope is between the 2 places, the second cannot be seen. A rough cross-section sketch shows this more easily. Intervisibility can also be affected by the presence of buildings or vegetation.

Originally published at https://geographypoint.com on July 3, 2015.