Elementary

SurveyingLecture : 5

By

Engr. Rafia Firdous

Department of Civil Engineering

University of Management and Technology Lahore

1

Compass Traversing

2

Introduction and Purpose Chain survey can be used when the area to be surveyed

is small and fairly flat.

But when the area is large, undulating and crowded withmany details, triangulation (principle of chain survey) isnot possible. For such areas method of traversing isadopted.

In traversing, the framework consist of a number ofconnected lines. The lengths are measured by chain ortape and the directions are measured by anglemeasuring instruments. One of the angle measuringinstrument is “Compass”.

The process in which directions are determined withcompass and lengths are measured with chain or tape isknown as “Compass Traversing”.

3

Meridian

Definition of meridian: “Any reference direction toobserve the bearing of survey line is called meridian”.

The bearing of a line is the horizontal angle which itmakes with the reference line (meridian).

4

5

True Meridian A line or plane passing through the

geographical north andgeographical south pole and anypoint on the surface of earth isknown as “true meridian”.

The true meridian is constant at astation.

The true meridian passing throughdifferent points on the earth’ssurface are not parallel butconverge toward the poles.

But for small survey areas theyare assumed to be parallel.

The angle between the truemeridian and a line is known as“true bearing of line or azimuth”.

Magnetic Meridian

When a magnetic needle is suspended freely and balanced

properly, unaffected by magnetic substances, it indicates

a direction, known as “Magnetic Meridian”.

The angle between the magnetic meridian and a line is

known as the “Magnetic bearing” or simply bearing of the

line.

6

Arbitrary Meridian Sometimes, for the survey of small area, a convenient

direction such as starting line of survey is assumed as

meridian, known as “Arbitrary Meridian”.

The angle between the arbitrary meridian and a line is

known as “Arbitrary Bearing”.

Sometimes state agencies carrying survey assumes several

lines parallel to true meridian for particular zone. These

lines are termed as “Grid Lines” and the central line is

known as “Grid Meridian”.

7

Grid Meridian

System of Magnetic Bearings Magnetic bearings are designated by two systems:

1. Whole Circle Bearing (WCB)

2. Quadrantal Bearing (QB)

The magnetic bearing of a line measured clockwise from the north pole

towards the line is known as the “whole circle bearing” of the line.

Such bearing have value between 0° to 360°.

WCB is obtained by Prismatic Compass.

8

1. Whole Circle Bearing (WCB)

9

The magnetic bearing of a line measured clockwise or counter

clockwise from the north pole or south pole towards the east or west is

known as the “quadrantal bearing” of the line.

This system consists of four quadrants “NE, SE, NW, SW”.

Such bearing have value between 0° to 90°. And the quadrant should be

mentioned.

QB is obtained by Surveyor’s Compass.

2. Quadrantal Bearing (QB)

Reduced Bearing When the whole circle bearing of a line is converted to

quadrantal bearing, it is termed as Reduced Bearing.

Hence the quadrantal bearing and reduce bearing are

same.

10

Fore and Back Bearing Every line have two bearings.

1. Fore Bearing

The bearing observed along the progress of the survey line or forward

direction is called Fore Bearing.

2. Back Bearing

The bearing observed reverse or opposite direction is called back

bearing.

Fore bearing and back bearing differ by 180°.

11

12

Magnetic Declination

13

14

Principle of Compass Surveying The principle of compass surveying is “traversing”.

Traversing involves series of connected lines. The

magnetic bearings (angles) of lines are measured by

Compass and the distances of the line are measured by

chain.

Interior details are located by taking offsets from the

main survey line. Sometimes subsidiary lines are also

taken.

Compass surveying is not recommended for areas where

local attraction is suspected due to presence of

magnetic substance like steel structures, iron ore

deposits, electric cables conveying currents etc.

15

Traversing Series of connected lines is known as traverse.

The sides of traverse are known as “traverse leg”.

There are two types of traverse “Closed and Open”.

1. Closed Traverse:

When the finishing point coincide with the start point.

Suitable for the survey of boundaries of ponds, forests, estate etc.

2. Open Traverse:

When start and end point are not same.

Suitable for survey of roads, rivers, coast lines etc.

Some of interior angles of traverse = (2n – 4) x 90°, n = number of sides

16

Compass & Types of Compass A compass is a small instrument essentially consisting of

magnetic needle, a graduated circle and a line of sight.

The compass can not measure angle between two lines

directly but can measure angle of a line with reference to

a magnetic meridian at the instrument station point,

called magnetic bearing of a line.

The angle between two lines is then calculated by getting

bearing of these two lines.

There are two types of compass available:

1. Prismatic Compass

2. Surveyor’s Compass

17

1. Prismatic Compass In this compass magnetic bearing

readings are taken with the help of a

prism.

It consists of circular metallic non-

magnetic compass box of diameter 8 –

10 cm, magnetic needle, graduated

ring, sight vane, prism, dark glasses,

brake pin, lifting pin, glass cover and

adjustable mirror.

The ring of compass is graduated from

0 to 360°, and the graduation begins

from the south end of the needle.

The magnetic needle is made of broad

magnetized iron bar.

The magnetic needle is attached with

the aluminium ring.18

19

1. Surveyor’s Compass

20

Surveyor’s compass is similar to the prismatic compass

except that,

There is no prism in it, readings aretaken with naked eye.

In place of prism there is an eyevane.

The graduated aluminium ring isattached to the circular box. And isnot fixed to the magnetic needle.

The ring is graduated from 0 to 90°

in four quadrants. 0° is marked onthe north and south and 90° ismarked on east and west.