Types of Section

Simple section

This is a simplest section as seen in the diagram below

 b) Annotated section

This is the same as the simple section, but the positions of important places and features, such as rivers or roads are marked. The positions of these places and features are marked on straight edges of paper at the time you are marking the contours. This can be seen in the diagram below.

c) Sketch Section

This is quicker to draw than an accurate section. You do not mark every contour on the edge of your paper.

You only mark contours representing the tops of hills and the bottom of valleys.

 River profile: This is a section taken along the course of the river.

 

The steps taken in drawing a cross section.

1.   Join the two points on the map using a sharp pencil. The end points of the cross section must always be shown. See the diagram below. (A and B)

       Hilly highland with two summits

2.         Take a piece of paper with a straight edge  which is slightly longer than AB. Place the edge of the paper along the line joining two points A and B. Mark points A and B on the piece  of paper. Hold the paper firmly to the line AB, and then working from A, make a vertical mark on the paper where every contour line meets the line AB. Give each of these marks its value as shown in the diagram below.

 

                        Marking the contour points

 3.   Draw a horizontal line the length of AB and mark the ends A and B. This forms the baseline which represents sea-level O feet.

4.   Draw two vertical lines, one on either side of the baseline. They should not be more than 6 centimeters in length. This is just a guiding principle to ensure that the cross section is not exaggerated.

5.   Find the vertical scale (VS).  In finding the V.S identify the highest and the lowest contour values. To determine the vertical scale, two things must be taken into account i.e in the final section:

So the vertical scale should be

350-0 = 350ft

350f = 6cm

1cm =350

            6

       1cm: 58.3ft

Or  1cm : 17.49m

      1cm : 1749cm.

6.  After marking all contours on your edge of paper, transfer the paper to your baseline. Using the vertical scale, transfer the readings from the edge to points vertically above them on the appropriate altitude lines marking the points with dots.

7. Join up the points with a smooth curve (do not use a ruler) starting from the beginning point (A) and ending at B which is the ending point, taking account of notes on the paper edge.

8. Shade the area of the Cross Section well. If it is not shaded, it is regarded as a line graph but not a cross section.

 

Vertical  exaggeration

Vertical exaggeration is the number of times the horizontal scale has been enlarged or increased  to obtain the  vertical scale. Before drawing a section between two points you must have a convenient vertical scale. The horizontal scale is the true scale of map. Normally the vertical scale is larger than the horizontal scale.

 Steps of calculating the vertical exaggeration.

Write the vertical scale as a ratio i.e.

1cm :58.3ft

1cm     : (58.3x0.3)m

1cm     :17.49m

1cm     : (17.49x100)cm

1cm     :1749 cm

1           : 1749

Convert the scales into a representative fraction.

 

 Vertical scale = 1

                          1749

 

Horizontal  scale      = 1

                                 50,000

 

V.E =      Vertical scale

            Horizontal scale.

V.E  = 1          1

        1749 ÷ 50,000

 

V.E = 1    x 50,000

         1749       1

 

V.E  = 50,000  =  29.

            1749  

 

Note:  The choice of a suitable V.E largely depends upon map scale and the type of country. It must be learnt by experience.

 The complete section should carry:

Suggested vertical exaggeration for various amplitudes of the 1:50,000 map series

 

Relief	Amplitude  in metres	V. E
Very flat plains	100m	6-20
Plateau and  low hills	100-250m	4-6
Dissected relief	250-500m	2-4
Very hilly areas	1,000m	1.25-2.5

 

Horizontal equivalent ( H.E )

The term horizontal equivalent is often used when dealing with slopes. The distance between two points on the surface when projectile on to a horizontal  equivalent. This can be shown in the a diagram below.

What is the distance of XY (H.E)

 

INTERVISIBLITY

This is to know, from a map if one place can be seen from another. To determine this the following steps can be  followed:

(a) Drawing a cross section between two places.

On the section, join the two places by a straight line from one vertical scale line to the other.

If the whole ground surface in the section falls on or below the straight line, the two places are intervisible.

 (b) The other way of knowing this is

 Calculating the Gradient between Two Places on A Map.

Gradient is the steepness of the slope between two places. The gradient of the slope is obtained by measuring the distance between two places (horizontal equivalent (HE) and their heights difference (vertical height )

Horizontal equivalent is the ground distance measured on a map.

Vertical height is calculated from contour valves and spot heights.

The vertical height and horizontal equivalent must be expressed in the same units. For example:

Taking two places on a map A and C, A is at the height of 900 metres and C 600 metres and the horizontal distance (H.E) between the two places is 4 km then the gradient will be

Gradient =   horizontal equivalent

                       Vertical height

  To calculate the gradient between A and C, the H.E must be converted in metres as the vertical height.

 

The vertical height which is AB = difference in height of the two places.

AB = 900-600 = 300m

HE = (4x1000) metres

V.H = 300 metes

Gradient = 4000  = 40 =13.33.

                   300       3   

Approximately 13 which is expressed as a ratio of  1:13 or a gradient of "1 in 13". This means that as one travels from C towards place A for every 13 metres travelled, the ground height rises by 1 metre.