S6 UNIT 1
Overview
Key unit competence: By the end of this unit, Learners should be able to draw cross sections and sketch maps by reduction or enlargement.
CROSS SECTION AND SKETCH MAPS
1.1. Contour, contour interval and importance of contours
A contour is a line drawn on a map joining all the places with the same height above sea level.
Contours cannot cross each other because each has its own fixed height and they can be close to one another in case of steep slopes. Contours are drawn at constant intervals known as the contour interval (CI). This is also called Vertical Interval (VI).
Contour interval is the difference in height between two adjacent contour lines.
contours are important to analyze the landforms:
- They help to identify landforms like hills, plateaus, mountains and valleys on a topographic map
- Hills/mountains on a topographic map are shown as concentric rings of contours with the highest values in the middle.
- In cases where contours are very close to each other, the relief of the area is steep.
-When contours are separated by a wide space that shows a flat land.
-It helps cartographers(teachers and students) to draw cross sections
1.2. A cross-section
A cross section is a topographical profile drawn between two points taken along a straight line. It normally shows changes in relief of the area indicated by two points on a topographic map.
To draw a cross section, the following steps are followed:
- Determine the beginning and the end points of the section.
- Draw a straight line lightly in sharpened pencil from one point to another.
- Place a piece of paper with a straight edge along the pencil line.
- Mark the contour values and other important information like rivers, settlements, roads, on the paper.
- Transfer the information on the paper to your graph paper and mark the point
on the cross section.
- Construct a frame with two vertical lines whose baseline is equivalent to the length of the line between the two points marked on the map.
Join all the points with a curved line following the dots on the paper to decide the bends of the line.
An example of drawing a cross section is given on the map below. The area of study is represented by the line between point A and point B.
To do this exercise, consider the following steps:
- Place a piece of paper on a map above along the line marked A and B
- Mark all the contours heights on the paper as shown below
- Draw two vertical lines at each end of the horizontal line, these will represent the Y -axes where the mark off the vertical scale will be made.
- Label point A and point B on the other end of the horizontal axis.
- Use the information on the paper to mark the highest and lowest heights marked along the line A and B.
- Label the horizontal axis by writing the horizontal scale
- Label the vertical axis by writing the vertical scale.
- Write a title of the cross-section: cross-section from point A to point B
- Join all the dots on the graph using a free-hand line.
- Make sure the cross-section line links to points A and B on the vertical axes.
1.3. Determining vertical exaggeration, the gradient, amplitude Aand inter-visibility
1.3.1. The vertical exaggeration
Refers to the relationship between the vertical scale and horizontal scale. It is calculated after drawing a cross section.
It is calculated by vertical scale over horizontal scale on the drawn cross section between two points
- To determine the vertical scale, refer to the scale given when asked to draw a cross-section, e.g. 0.5 cm represents 20 m. This scale needs to be converted into centimeter units to be used in the formula.
0.5 cm = 20 m
0.5cm = 2000cm
5cm = 20,000cm
1cm = 4000cm
Therefore, 1cm represents 4000 cm
- On topographic maps, the horizontal scale is most of time the same 1: 50,000,
i.e. 1 cm represents 50 000cm.
- As all units have been converted into centimeters, insert these measurements into the formula.
1.3.2. The gradient
Gradient refers to the steepness of a slope between two places.
It is expressed as a proportion between the vertical intervals (VI) and horizontal equivalent (HE).
Vertical interval or vertical rise (V. I). Is the the difference in height between two points
The horizontal equivalent or real distance on the ground (H.E) is the horizontal distance between the two places multiplied by the scale.
Gradient is calculated as follows:
- Plot the two points on the map which are needed to determine the gradient.
Name them for instance as A and B or X and Y.
- Join those two points by a straight line.
- Use the scale to measure the distance between A and B (H.E). Let us consider the distance to be 8 cm.
- Calculate the actual ground distance using the map scale. If the scale of the map is 1:50,000 meaning that 1 cm on the map represents 50,000 cm on the ground. Therefore, the ground distance of the represented area would be 8 x 50,000 which 400,000 cm = 4000 m.
- Calculate the difference in height between points A and B, using the contours.
For example, the difference in height between A and B is 200 m.
- The formula for calculating the gradient is:
1.3.3. Amplitude
Amplitude: This refers to the difference between the highest points (altitude) on the map and the lowest altitude on the map
Inter-visibility: Inter-visibility is a term that refers to the ability to see a place from another place. For example, if one is able to see place A from B, and vive versa, then the two places are intervisible. On the other hand, if the two places cannot be viewed from either side, there are not intervisible.
The figures below will make these explanations.
There is intervening obstacle in the line of visibility and therefore points A and B are not intervisible.
Dead ground is the section of the slope that is not visible.
1.4 Drawing sketch maps:
A sketch map is a simple representation of part or whole of a sheet map drawn on a piece of paper without using a given scale.
Procedure:
- Identify and critically observe the area to be sketched on the map given.
- Measure the edges of the map.
- Make a frame by either reducing or enlarging the map as instructed.
- Indicate both physical and human features as requested.
- Give the name of the sketch
- Provide the key for the sketch map.
1.5. Enlargement and reduction of a map
1.5.1. Map Enlargement
Map enlargement is a change of size of the map from a small to a bigger size map, depending on the number of times it is enlarged
The following steps should be followed:
- Identify an area of the original map or part of the map to be enlarged.
- Measure the length and width of the original map or identified part of the map.
- Multiply the length and width by the number of times the map is to be enlarged.
- Draw an outline that has new dimensions.
- Mark and label the features in their relative positions.
- The scale also changes (becomes bigger).
The following map of Africa has been enlarged by 2 times from the above map
1.5.2. Map reduction
Map reduction refers to the changing of the size of the map to a smaller one
the steps to follow for map reduction:
- Measure the length (L) and width (W) of the given part or whole map. For example, L=11cm and W=10.8cm.
- Divide the length and width by the number of times the given map is to be reduced or as directed by the demands of the question. For example, by 2 times.
- Draw an outline that has the new length and width. For example, L=5.5cm, W=5.4cm.
- Mark and label the features in their relative positions.
- Use a key to label features in the map.