## HOW TO DRAW CROSS-SECTION FROM TOPOGRAPHICAL MAP

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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## How to do map enlargement and map reduction

### IN REDUCING ANY MAP TO A GIVEN SIZE, THE FOLLOWING STEPS SHOULD BE FOLLOWED:

• Measure the length and width of the map.
• Divide the length and width by 2 or 4 if you are asked to reduced the map to half or a fourth of its original size. For example, if the length and width of a map are 24cm and 20cm respectively, such a map should measure 12cm by 10cm if seduced to half its size and 6cm by 5cm if reduced to a fourth of its size and so on.

• Having reduced the original map, it is obvious that the scale would equally change. Therefore, if a map has a scale of 1:50,000, the scale of the map changes to 1:100,000 if the size has been reduced to half, and 1:200,000 if the size of the map has been reduced to a fourth of its original size.
• The features to be shown on the reduced map should also be proportional to the required size of the map.
• When you have finished drawing the reduced map, remember to write its title and the new horizontal scale

### IN ENLARGING ANY MAP THE FOLLOWING STEPS SHOULD BE TAKEN

• Measure the length and width of the original map.
• Multiply the length and width by 2 or 4 respectively if you intend to enlarge the map to twice or thrice its original size.For example, if the length and width of a map are 5cm and 3cm respectively, such a map would measure 10cm by 6cm if enlarged to twice its size and 20cm by 12cm if enlarged four times its size and so on.
• Having enlarged the original map, it is equally obvious that the scale would change. Therefore, if a map has a scale of 1:60,000, the scale of the map changes to 1:30,000 if the size has been enlarged twice and 1:15,000 if the size of the map has been enlarged to four times its original size.
• The features to be shown on the enlarged map should also be proportional to the required size of the map.
• When you have finished drawing the enlarged map, write its title and the new horizontal scale.

Before one can know whether to reduce or enlarge a map, it may be necessary to divide the initial scale (scale of the original map) by the scale of the new map to be drawn.

For example, if the scale of a map is 1:50,000 and is to be reproduced to a scale of 1:200,000, divide the former scale with the latter as follows:

50000/200000=1/4 Therefore the new map should be 1/4 times the size of the original map.

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## HOW TO DESCRIBE CLIMATE IN TOPOGRAPHICAL MAPS

Climate is the average weather condition of a given area recorded over a very long period of time.

The topographic maps show little direct evidence of the climatic conditions. yet by looking at certain evidence one is able to describe the climate of the given mapped area.

The description of climate in the topographic map is based on two-element of climate namely temperature and rainfall since their variation can easily be explained by various evidence from the map.

## Some hints in interpreting climate from topographic map:

### Latitude of the area

latitude is the angular distance of a place north or south of the earth’s equator, or of a celestial object north or south of the celestial equator, usually expressed in degrees and minutes.

Through studying latitude one is able to deduce the climatic condition of a given mapped area.

For example, in the area with a latitude 5 degrees north or south of the equator the area is considered to be in equatorial climatic condition while the area between 5 to 15 degrees north or south of the equator is in tropical climatic condition.

### Presence of water bodies and drainage pattern

A general high density of streams indicates that the area receives high rainfall. in another case the presence of low density, intermittent or seasonal streams, salt lakes, and boreholes indicate aridity.

### Vegetation

The presence of forest in the map indicates heavy rainfall while woodland vegetation indicates moderate rainfall. shrubs, thickets, and grassland indicate a dry condition or light rainfall

### Crops

crops grow in those areas where water and temperature conditions are favorable for their growth.

For example area with tea, coffee, and sugar cane indicate that area receives heavy rainfall. Crops such as cotton, sisal, and sorghum indicate medium rainfall

### Relief/altitude

the altitude of features such as high mountain or mountain ranges and plateaus tend to have orographic rainfall and forest making them make their own mountainous climate.

Mountains do also create their own climate due to the effect of aspect which creates a rain shadow

## HOW TO DESCRIBE VEGETATION DISTRIBUTION IN TOPOGRAPHICAL MAP

Vegetation is the collective name for plants in particular area. it can be natural (man have not planted them) or artificial(planted by man)

The natural vegetation can be categorized into

Forest

Grassland

Marshy and swampy vegetation
Thickets
Riverine trees
Woodland

Scrubs

In the topographic map, the distribution of natural vegetation is shown by conventional symbols. This means they are all listed on the map key.

## The following are hints on how to describe vegetation in topographic map

• first, categorize or group the type of vegetation as shown in the map. So this means that you have to look in map key to see symbols representing vegetation after that yo will look into the map and state what type of vegetation you have found. For example if you see forest and woodland in your map then state that in the map provided vegetation consist of forest and woodland

• Secondly, give brief explanation to vegetation distribution. This means you have to state using direction, place names or any other methods of locating place on map the location of each type of vegetation you identified earlier.

## what is drainage

Drainage is the outflow of water through the system of natural rivers

## what is drainage pattern?

A drainage pattern is a layout that a river and its tributaries, or set of rivers, make on the earth’s surface. Some of the common river drainage patterns include dendritic river pattern, trellised river pattern, radial river pattern and centripetal river pattern

## The following are hints to follow when you describe drainage in topographical map

• Divide the map into drainage basins using compass directions. Note that the drainage basin is an area drained by single river system.
• In each drainage basin explain the drainage pattern which the river system in the basin take. The common river drainage pattern are
• For each drainage basin explain how the rivers drain the relief region i.e highlands, lowlands and plateaus. at this part state the direction of the flow of the river, where the river is pouring its water and whether the area is well drained or not. The area is well drained when there is no swamps or marshlands and is not well drained when is waterlogged areas.