**A cross-section** is a profile of the area under discussion. It is a diagram showing the 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 have an idea of the shape BEFORE you begin plotting heights or drawing the section.
- Join the 2 points, making sure the FROM point is on the left.
- 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. Eg. 2 2 2 2 0 2 4 6 0 0 0 0
- Now place the “magic” piece of paper along the base line 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 eg. Cross section from • 96 to r 281 on 3326 BC Grahams 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** (1cm represent 50000cm)

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.

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