Surveying Study Notes — Class 12 Geography

Lesson summary

Unit 9 opens Practical Geography. It defines surveying, shows how to measure the height of a tall object with a clinometer, and describes the parts and uses of the prismatic compass.

9.1 Introduction

Surveying has been an essential element in the development of man's environment for centuries.
It is important in planning and executing every form of construction.
It is used to map the earth above and below the sea, prepare navigational maps and establish databases for natural resource management.
It develops engineering data for huge buildings, settlements, roads, railways and bridge constructions.
Definition: surveying is the science of measuring and recording distances, angles, heights and sizes of the earth's surface to obtain data from which accurate plans and maps are made.
Generally, surveying is the systematic process of making measurements in the field from which maps are drawn.
The map is the most essential piece of equipment which the geographer, engineer and architect use. Geographers must learn to read, make and use maps.

9.2 Clinometers

A clinometer is used to measure the height of a tall object such as a tree or an electric or telephone pole.

What you need: clinometers, tape, paper, pen or pencil, and an assistant.

Step	What to do
1. Pick a spot	Stand far enough away to see the top of the object, on level ground with its base.
2. Measure angle	Look through the straw at the top; the weighted string hangs freely across the protractor. Subtract the reading from 90°. Example: 55° read, so angle of vision = 35°.
3. Measure distance	Use a tape to measure the distance to the base of the object. Example: 15.6 m.
4. Find your eye-height	Measure the height from the ground to your eye. Example: 1.64 m.
5. Draw a picture	Sketch the situation and label it with all the information.
6. Model as a triangle	Draw a right triangle with the angle and the distance. Eye-height is not needed yet.
7. Combine with eye-height	h = x + (eye-height).

Working with the tangent ratio

In the triangle, x is the portion of the height from eye-level up.
tan(angle) = x / distance, so x = tan(angle) × distance.
Height of the tree = BC + CD, where CD is the height of the observer and AC is the distance between the observer and the tree; BC = AC tan(90 - Z), Z being the clinometer reading.
Example: tan 35° = 0.7002, so x = 0.7002 × 15.6 = 10.92 m.
Then h = 10.92 m + 1.64 m = 12.56 m.

9.3 Prismatic compass

A simple instrument with an aluminium ring graduated to 30 minutes, held at the top of a broad magnetic needle balanced on a hard steel pivot.
The compass sits in a circular metal box with a transparent glass cover, which is covered by a hinged metal lid.
On one side is a hinged eye-vane with a prism that magnifies the degree marking on the aluminium ring, reflected from the hypotenusal side of the prism.
Two coloured sunglasses at a hinge avoid direct sun rays at the eye vane.
On the other side is a sighting vane: a hinged metal frame with a horse hair at the centre, which in reality may be a fine silk thread or a metal wire.
A hinged adjustable mirror on the outer side of the sighting vane locates a too high or too low object through reflection.
A brake pin or knob brings the oscillating circular ring to a standstill.
The threaded bottom of the metal box screws onto a knob on top of a ball, adjustable in a socket of the compass stand (ball and socket arrangement for levelling).

Part (diagram label)	Function
Prism with sighting slit	Magnifies the degree marking on the ring
Sun glasses	Avoid direct sun rays
Sliding mirror	Locates very high or low objects
Object vane	Holds the horse hair used for sighting
Graduated aluminium horizontal ring	Shows the angle, graduated to 30 minutes
Magnetised needle on a pivot	Carries the ring
Ball and socket arrangement	Levelling

Use of prismatic compass

Measures the horizontal angle with reference to the magnetic north line.
The bearing obtained is a magnetic bearing, measured clockwise from magnetic north.
Corners of a plot or building and bends of a road can be measured directly.
Bends of a meandering canal, divided into segments, can be favourably determined.

Surveying - Study Notes