Geography · Lesson 20

Practical geography 5: chain and compass survey, levelling, GPS, remote sensing and GIS

WAECNECO

What the syllabus asks

WAEC: elementary surveying (chain and prismatic compass); Geographic Information System

NECO: elementary surveying (chain and prismatic compass); Geographic Information System

What you will learn

Describe chain-survey equipment and procedure.
Book offsets and calculate areas by the trapezoidal rule and Heron’s formula.
Measure and check bearings with a prismatic compass.
Plot a traverse and calculate its closing error and accuracy.
Reduce levels by rise-and-fall and height-of-collimation methods.
Explain GPS and remote sensing and their uses in Nigeria.
Describe GIS components, data models and functions.
State problems of GIS in Nigeria and suggest solutions.

Key points

Chain survey measures distances; triangles fix shape.
Book chainages upwards; offsets left or right.
Trapezoidal rule: interval × (half ends + middles).
Compass bearings from magnetic north; back bearing ± 180°.
Unequal FB/BB difference shows local attraction.
Closing error and accuracy check a closed traverse.
Levelling: rise-and-fall with arithmetic check.
Remote sensing: passive, active, resolution, NigeriaSat.
GIS: five components; vector and raster; layers.
Problems of GIS in Nigeria: cost, skills, power, data.

Start here: Concept summary: surveying and geographic information

Surveying is the measurement of the positions, distances, directions and heights of features on the ground so that they can be mapped. The WAEC and NECO syllabus requires elementary surveying: chain and prismatic compass surveys, examined in Paper 3 as one of the optional 15-mark questions, and JAMB lists elementary surveying and Geographic Information Systems (GIS) in its practical-geography section. Questions ask you to describe the equipment and procedure, book or plot field data, calculate areas or errors, and state advantages and disadvantages.

This unit covers: chain survey (equipment, procedure, booking, offsets, plotting, area); prismatic compass survey (bearings, back bearings, traverses, closing error, local attraction); levelling (included for completeness, since it is taught in many schools and occasionally asked, but it is not named in the WAEC syllabus summary; see the syllabus log); GPS; remote sensing; and GIS concepts, components, data sources, uses and problems in Nigeria.

All field data in this unit are illustrative and every calculation has been checked by computer. Unit gg9 gives the basic descriptions; here the emphasis is on doing the calculations and plotting the way the practical paper requires.

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Practice questions

1. In chain surveying, short measurements at right angles from the chain line are

offsets
bearings
tie lines
stations

Show answer

A: offsets
To features.

2. Ranging poles are used to

measure angles
measure height
record offsets
keep survey lines straight

Show answer

D: keep survey lines straight
Line out the chain.

3. A tie line is used to

find north
plot contours
check the accuracy of a chain survey
measure height

Show answer

C: check the accuracy of a chain survey
Checks the framework.

4. A prismatic compass is used to measure

rainfall
bearings
heights
distances only

Show answer

B: bearings
Magnetic bearings.

5. A difference between forward and back bearings other than 180° indicates

local attraction
a good survey
a long line
levelling

Show answer

A: local attraction
Iron or magnetic objects nearby.

Key words

Chainage: Distance along a chain line from its start.
Offset: Perpendicular distance from a chain line to a feature.
Ranging: Lining up poles so a chain line is straight.
Well-conditioned triangle: Triangle with no angle below about 30° or above about 120°.
Prismatic compass: Compass with a prism for reading bearings while sighting.
Local attraction: Deflection of a compass needle by nearby iron.
Traverse: Connected survey lines of measured direction and length.
Closing error: Gap between start and end of a plotted closed traverse.
Bench mark: Point of known height used in levelling.
Remote sensing: Gathering data about the earth from a distance.
Resolution: Smallest object distinguishable in an image.
GIS: Computer system for capturing, analysing and displaying spatial data.

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