Prismatic Compass Survey Chapter 2

PRISMATIC COMPASS SURVEY

1.0 Introduction

Compass is always showing the magnetic poles. It is a wonderful direction finder. The earth’s
magnetic field and the use of the compass have been known to navigators and surveyors for many
centuries. In fact, before the sextant and transit were developed, the compass was the only means
by which the surveyor could measure angles and directions.

A surveyor uses a compass to determine the direction of a line. The compass

needlepoints to the MAGNETIC NORTH POLE and by turning the compass in the
direction of the line being surveyed, the direction of the line can be observed.
Although there are many varieties of compasses, they all fall into two main

Categories: either a "plain" compass or a "vernier" compass.

A plain compass has no adjustment and always reads magnetic north.

A vernier compass has an adjustable scale that allows for the "setting off" of the magnetic
declination and the compass can then directly read true north.

Today, the magnetic north pole is located approximately 1000 miles south of the astronomic
North Pole in the Canadian artic near Bathurst Island. The compass needle lines up with magnetic
north. ; In most places this means that the needles points slightly east or west of astronomic north,
depending on the locality. The angle between astronomic north and magnetic north is referred to as
the magnetic declination, which is 40 minute deflected.

Astronomic
North

Magnetic
Grid North North

1000 miles

35’ 40’

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Magnetic meridians are lines parallel to the directions taken by freely moving magnetized needles,
as in a compass.
A geographic meridian is a line on the earth joining the north and south poles. It also known
as a astronomic or ‘true’ meridian.
Grid meridians are lines that parallel to a grid reference meridian.

Meridians are important to the surveyor because they are used as a reference direction for
surveys. All lines can be related to each other and to the real world by angles related to
meridians. These angles are called bearings and azimuths.

2.0 Bearings

A bearings is the direction of a line given by a cute angle between the line and a meridian.
The bearing angle can be measured clockwise or counterclockwise from the north or south
end of a meridian, is always accompanied by the letters that describe the quadrant in
which the line is located (NE,SE,SW and NW). Bearing angle can be range from 0˚ to 90˚
degree.

N
48º 00’00”
y

NW NE

W x E

SW SE

Notes:
ie// x – y : N 48º 00’00” E

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3.0 Azimuths

An azimuth is the direction of a line given by an angle measured clockwise from the north
end of a meridian. Azimuths can be range in magnitude 0˚ to 360˚ degree.

360 / 0

48º 00’00”
y

270 90
x

Notes :
ie//x-y : 48º 00’00”

180

4.0 Bearing Adjustment Method

There are three adjustments for bearings: -
i. An average of forward bearings
ii. By internal angle
iii. Local attractions

4.1 An average of forward bearings

Example: -

Forward bearing AB = 48º 00’00”

Reverse bearing AB = 229º 00’00”
Differences the two bearings is 181º 00’00”
It’s supposed to be 180º 00’00” , obviously, an adjustment is:

By using a forward bearing as a reference,

Forward bearing observed = 48º 00’00”
Forward bearing compute = 229º 00’00” - 180º 00’00”
= 49º 00’00”

An average of forward bearing = (48º 00’00” + 49º 00’00”) / 2

= 48º 30’00”

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4.2 Internal angle method

Example: -

Stn Lines Observed Diff Observed C’tio Corrected Final New

Bearing Internal n Internal Bearing Diff
Angle Angle
A A-B 48º 00’00” 048º 18’00”
181 87º 00’00” - 42’ 86º 18’00” 180
B-A 229º 00’00” 228º 18’00”
B-C 118º 30’00” 110º - 42’ 118º 30’00”
180 109º 48’00” 180
C-B 298º 30’00” 30’00” 298º 30’00”
C C-D 163º 15’00” 135º - 42’ 163º 57’00”
181º 45’00” 134º 33’00” 180
D-C 349º 00’00” 15’00” 343º 57’00”
D D-E 252º 00’00” - 42’ 247º 39’00”
179º 15’00” 97º 00’00” 96º 18’00” 180
E-D 72º 45’00” 067º 39’00”
E E-A 319º 00’00” 113º - 42’ 314º 36’00”
184 113º 03’00” 180
A-E 135º 00’00” 45’00” 134º 36’00”
 = 543º 30’00”  = 540º
Computation of observed internal angle

Stn A = 135º 00’00” - 48º 00’00”

= 87º 00’00”

Stn B = 229º 00’00” - 118º 30’00”

= 110º 30’00”

Stn C = 298º 30’00” - 163º 15’00”

= 135º 15’00”

Stn D = 349º 00’00” - 252º 00’00”

= 97º 00’00”

Stn E = (360º 00’00” - 319º 00’00”)+ 72º 45’00”

= 113º 45’00”

 Observation of internal angle = (2n-4) 90º

where n is the numbers of boundaries.

Therefore = (2 (5 ) − 4) 90º
= 540º

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 Observation of internal angle = 87º 00’00” + 110º 30’00” + 135º 15’00”

+ 97º 00’00” + 113º 45’00”
= + 543º 30’00”

Errors = 540º - 543º 30’00”

= - 3º 30’00”

Therefore, an angle adjustment = - 3º 30’00”/ 5

= - 00º 42’00” contributed to 5 stations.

4.3 Local Attractions

The direction taken by a compass needle is affected by magnetic attractions other than
that of the earth’s magnetic field. Fences, underground pipes, reinforcing bars, passing
cars, nearby buildings, iron ore deposits under the earth’s surface, and other steel or iron
objects may have a considerable effect on compass reading. In addition, the effect of
power lines, particularly because of variations in voltage, may be so great compasses
are useless in some areas. On many occasions the surveyor may not realize that the
magnetic bearings reading with the compass have been affected by local attractions.
To detect local attraction, reading must be read both forward and back bearings for
each line to ensure if they correspond reasonably well.
"Local magnetic attraction" means the deflection of the compass needle by a local
magnetic force, such as that created by nearby electrical equipment or by a mass of
metal, such as a bulldozer. When local attraction exists and is not compensated for, the
bearing you get is a COMPASS bearing. A compass bearing does not become a
magnetic bearing until it has been corrected for local attraction. Suppose, for example,
you read a compass bearing of N37?E. Suppose the effect of the magnetic attraction of
a nearby pole transformer is enough to deflect the compass needle 4? to the west of
the magnetic meridian. In the absence of this local attraction, the compass would read
N33?E, not N37?E. Therefore, the correct magnetic bearing is N33?E.

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Example: -

Local Attraction Corrected New Final

Stn Line Bearing Distance Different
Correction Bearing Different Bearing

A-B 309˚30’00” + 1˚30’00” 311˚00’00” 311˚00’00”

A 90 177˚00’00” 180˚
- 1˚30’00” 131˚00’00” 131˚00’00”
B-A 132˚30’00”
B-C 54˚00’00” - 1˚30’00” 52˚30’00” 52˚30’00”
B 80 179˚00’00” 180˚
- 0˚30’00” 232˚30’00” 232˚30’00”
C-B 233˚00’00”
C-D 176˚30’00” - 0˚30’00” 176˚00’00”
C 176˚00’00”
60 179˚30’00” 180˚
0˚00’00” 356˚00’00”
D-C 356˚00’00” 356˚00’00”
D-E 138˚30’00” 0˚00’00” 138˚30’00”
D 138˚30’00”
100 180˚00’00” 180˚
0˚00’00” 318˚30’00”
E-D 318˚30’00” 318˚30’00”
E-A 259˚30’00” 0˚00’00” 259˚30’00”
E 259˚30’00”
70 181˚30’00” 180˚
+ 1˚30’00” 79˚30’00”
A-E 78˚00’00” 79˚30’00”

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Reflecting mirror

Objective vane with horse hair Hinged sun glass

Eye slit
Sliding arrangement

Lifting pin

Clamping ring Right angled convex prism

Glass cover
Eye Vane
Graduated Circle

Magnetic needle
Prism cap
Agate cap
Rider

Focussing stud
Pivot
Hinged strap
Brake pin

Spring brake Lifting lever Metal box

Sectional view of prismatic compass