12th Standard Geography — Surveying Online Test
Unit 9 of 12th Geography, the first Practical Geography lesson, explains surveying as the science of measuring distances, angles and heights. It shows step by step how to find the height of a tree with a clinometer using the tangent ratio, and describes the parts and uses of the prismatic compass.
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About Surveying
Medium ~35 min study
Surveying is Unit 9 of the Tamil Nadu State Board Class 12 Geography book and opens the Practical Geography section. It explains why surveying has been an essential element in the development of man's environment for centuries, from mapping land and sea to planning buildings, roads, railways and bridges.
The lesson then turns to two field instruments. With a clinometer you learn to find the height of a tree or pole in seven steps, using the tangent ratio and your own eye-height. With the prismatic compass you learn its parts and how it measures horizontal angles from the magnetic north line.
The textbook prints no one mark questions for this unit, so this page offers additional questions written from the lesson, covering definitions, the clinometer steps, simple height calculations and the prismatic compass, each with a short explanation.
What you'll learn
- Define surveying and explain its importance.
- List the things needed to measure height with a clinometer.
- Follow the seven steps of finding the height of an object.
- Use the tangent ratio to calculate height from angle and distance.
- Name the parts of a prismatic compass and state their functions.
- State the uses of a prismatic compass in the field.
Before you start
- Basic idea of angles measured in degrees and the use of a protractor.
- The tangent ratio of a right-angled triangle from mathematics.
- Reading and using maps from earlier Geography lessons.
Topics covered in this chapter
Surveying explained
What this lesson covers
Introduction to surveying
Surveying is the science of measuring and recording distances, angles, heights and sizes of the earth's surface. It is the systematic process of making field measurements from which maps are drawn. The map is the most essential piece of equipment for the geographer, engineer and architect.
Clinometers
A clinometer measures the height of a tall object. You pick a spot on level ground, read the angle through the straw and subtract it from 90°, measure the distance with a tape, measure your eye-height, draw a picture and model it as a right triangle. The tangent ratio gives x = tan(angle) × distance, and the height is h = x + eye-height. In the book's example h = 10.92 m + 1.64 m = 12.56 m.
Prismatic compass
The prismatic compass has an aluminium ring graduated to 30 minutes on a broad magnetic needle, a prism to magnify the degree marking, coloured sunglasses, a sighting vane with a horse hair, an adjustable mirror, a brake pin and a ball and socket for levelling. It measures horizontal angles with reference to the magnetic north line, such as the corners of a building or the bends of a road.
Common mistakes to avoid
- Using the clinometer reading directly. The reading must be subtracted from 90° to get the angle of vision (55° gives 35°).
- Forgetting the eye-height. The triangle gives only x, the height above eye-level; h = x + eye-height.
- Adding the eye-height while modelling the triangle in Step 6. It is added only in Step 7.
- Thinking the prismatic compass measures vertical angles. It measures horizontal angles from the magnetic north line.
- Taking the horse hair of the sighting vane literally. In reality it may be a fine silk thread or a metal wire.
Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.
Frequently asked questions
What is surveying?
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 it is the systematic process of making measurements in the field from which maps are drawn.
Why is surveying important?
Surveying 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, build databases for natural resource management, and develop engineering data for buildings, settlements, roads, railways and bridges.
What do you need to measure the height of a tree with a clinometer?
You need a clinometer, a tape, paper, a pen or pencil and an assistant. The assistant reads the clinometer while you look through its straw, and helps measure the distance to the tree and your eye-height.
How is the angle of vision found from a clinometer reading?
Look through the straw at the top of the object and let the weighted string hang freely across the protractor. Read the angle shown and subtract it from 90°. In the lesson the clinometer showed 55°, so the angle of vision was 35°.
How is the height of the tree calculated in the lesson's example?
With tan 35° = 0.7002 and a distance of 15.6 m, x = 0.7002 × 15.6 = 10.92 m. Adding the eye-height of 1.64 m gives the height of the tree, h = 10.92 m + 1.64 m = 12.56 m.
What does a prismatic compass measure?
A prismatic compass measures the horizontal angle with reference to the magnetic north line. The bearing so obtained is a magnetic bearing. Corners of plots or buildings and bends of roads can be measured directly, and meandering canals after dividing them into segments.
What are the main parts of a prismatic compass?
It has an aluminium ring graduated to 30 minutes on a magnetic needle inside a circular metal box with a glass cover and lid, a hinged eye-vane with a prism and two coloured sunglasses, a sighting vane with a horse hair and an adjustable mirror, a brake pin, and a ball and socket for levelling.
Last updated 19 October 2026