COMPASS SURVEYING

CONTENTSINTRODUCTIONINSTRUMENTS USED IN COMPASS

SURVEYINGPRISMATIC COMPASSIMPORTANT TERMS OF COMPASSTYPES OF NORTHADJUSTMENT OF PRISMATIC COMPASSOBSERVING BEARINGPROCEDURERESULTERRORS IN COMPASS OBSERVATIONSCONCLUSION

INTRODUCTIONSurveying is the art of determining the relative positions

of different object on the surface or beneath of the earth by measuring the horizontal distance between them and by preparing a map to any suitable scale. Thus, in this process, the measurements are taken only in the horizontal plane.

Compass survey is a method of surveying by taking bearings and linear distances to produce plan.

Bearing is measured using prismatic compass, while the linear distance is measured using measuring tape.

Cont....Bearing means the angle between a line connecting us

and another object, and a north-south line. (i.e. a meridian)

Compass surveying is recommended when:i) A large area is to be surveyedii) A course of a river or coast line is to be surveyed andiii) The area is crowded with many details and triangulation is not possible.

Compass surveying is not recommended for areas where local attraction is suspected due to presence of magnetic substances like steel structures, iron ore deposits, electric cables conveying current and so on.

INSTRUMENTS USED IN COMPASS SURVEYING

(1) (2) (3) (4)

(5)

(1) Prismatic compass on tripod stand(2) Ranging rods(3) Plumb bob(4) Measuring tape(5) U-fork

PRISMATIC COMPASS

PARTS OF PRISMATIC COMPASS

PRISMATIC COMPASS : DESCRIPTION

COMPASS BOX: It is a circular box of diameter 85 to 110 mm having pivot at the center and covered with plain glass at top.

MAGNETIC NEEDLE: It facilitates in taking the bearings of survey lines with reference to the magnetic north.

GRADUATED RING: The bearings are marked inverted on the graduated rings from 0° to 360° in a clockwise starting 0° from south.

PIVOT: Magnetic needle is freely held with this.

Cont...EYE VANE: It consists of prism with a sighting slit

at the top. The prism magnifies and erects the inverted graduations

BRAKE PIN: It is pressed to stop the oscillations of the graduated ring.

LIFTING PIN: On pressing it brings the lifting lever into action.

COLOUR GLASSES: Red and blue glasses are provided with the prism to sight luminous objects.

PRISMATIC COMPASS• Prismatic Compass comprises of a magnetic

needle attached to the circular ring made up of aluminum.

• The needle is on the pivot and will orient itself in the magnetic meridian Therefore the north and south ends of the ring will be in this direction.

• The line of sight is defined by the objective vane and the eye slit, both attached to the compass box.

• The object vane consist of a vertical hair attached to a suitable frame while the eye slit consist of a vertical slit cut in to the upper assembly of the prism unit, both being hinged to the box.

When an object is sighted, the sign vanes will rotate with respect to the N-S end of ring through an angle which the line makes with the magnetic meridian.

A triangular prism is fitted below the eye slit, having suitable arrangement for focusing to suit different eye sight.

The readings increase in clockwise direction from 0o at South end 90o at West end 180o at North end and 270o at East end.

The object vane frame can be folded on the glass lid which covers the top at box.

The object vane presses against a bend lever which lifts the needle of the pivot and holds it against the glass lid.

When bright objects are sighted dark glass may be interposed in to the line of sight.

IMPORTANT TERMS OF COMPASSMERIDIAN – it is the fixed or the reference direction in which

the bearings of survey lines are expressed.BEARING – it is horizontal angle between the reference meridian

and the survey line measured in clockwise or anticlockwise direction.

TRUE MERIDIAN - line or plane passing through geographical north pole and geographical south pole.

TRUE BEARING – The horizontal angle measured clockwise between the true meridian and the line is called true bearing of the line.

MAGNETIC MERIDIAN - When the magnetic needle is suspended freely and balanced properly, unaffected by magnetic substances, it indicates a direction. This direction is known as magnetic meridian.

MAGNETIC BEARING - The angle between the magnetic meridian and a line is known as magnetic bearing

cont…Designation of magnetic bearing

a) Whole circle bearing (WCB)b) Quadrantal bearing (QB)

• WCB: The magnetic bearing of a line measured clockwise from the North Pole towards the line is known as WCB. Varies 0-360°

Quadrantal Bearing: The magnetic bearing of a line measured clockwise or anticlockwise from NP or SP (whichever is nearer to the line) towards the east or west is known as QB. This system consists of 4-quadrants NE, SE, NW, SW. The values lie between 0-90°QB of OA = N a E

cont...Reduced Bearing: When the whole circle bearing of a line

is converted to quadrantal bearing it is termed as reduced bearing.

Fore and Back Bearing:

In WCB the difference between FB and BB should be exactly 180° BB=FB+/-180° Use the +ve sign when FB<180° Use the –ve sign when FB> 180°

Every line has two bearings one observed at each end of the line. The bearing of the line in the direction of progress of the survey is

called Fore Bearing (FB), while the bearing in the opposite direction is called Back Bearing (BB).

TYPES OF NORTH• True North: True north is the direction along the

earth's surface towards the geographic North Pole

• Grid North: The direction in which the grid lines point towards the top of map.

• Magnetic North: It is the direction in which the compass needle points towards the magnetic pole.

ADJUSTMENT OF PRISMATIC COMPASS

The following are the adjustments usually necessary in the prismatic compass:

• Centering• Leveling• Focusing the prism.

CENTERING:The center of the compass is placed vertically over the

station point by dropping a small piece of stone below the center of the compass, it falls on the top of the peg marking that station. Or by using plumb bob with u-fork.

LEVELLING:By means of ball and socket arrangement the Compass is

then leveled the graduated ring swings quite freely. It may be tested by rolling a round pencil on the compass box.

FOCUSSING THE PRISM :The prism attachment is slid up or down focusing till the

readings are seen to be sharp and clear.

OBSERVING BEARING• The compass centered over station A of the line AB and is

leveled. • Having turned vertically the prism and sighting vane, raise or

lower the prism until the graduations on the rings are clear and look through the prism.

• Turn the compass box until the ranging rod at the station B is bisected by hair when looked through the prism.

• Turn the compass box above the prism and note the reading at which the hair line produced appears to cut the images of the graduated ring which gives the bearing of line AB.

There are two systems commonly used to express the bearing.

WHOLE CIRCLE BEARING: In this system the bearing of a line measured with the magnetic north in clockwise direction. The value of bearing thus varies from 0o to 360o.

QUADRANTAL SYSTEM:In this system the bearing of a line is measured eastward or westward from north or south whichever is near. The directions can be either clock wise or anti clockwise depending upon the position of the line.

INTERSECTION METHOD TO PLOT THE GIVEN AREA BY USING PRISMATIC COMPASSPROCEDURE :• Two station points X and Y are to be fixed. • Then chain must be stretched along X & Y after ranging. Compass

must be fixed on the tripod and then centering and leveling to the point X is to be carried out.

• FB of XY is then to be taken. At point X, bearing to the five points (A, B, C, D) are to be taken and entered in the tabular column.

• Compass is then to be shifted to point Y. Centering and leveling are to be done. BB of XY and bearing of five points are then to be taken.

• Then calculate the Reduced Bearing (RB). With a suitable scale, the points are to be plotted or can be done with whole circle bearing also.** FB = Fore bearing** BB = Back bearing

SL. STATION SIGHTED TO W.C.B.

1. X A2. X B3. X C4. X D

SL. STATION SIGHTED TO W.C.B.

1. Y A2. Y B3. Y C4. Y D

TABULAR FORM

Schematic diagram of the surveyed area :

RESULT

The given area is plotted on the drawing sheet.• Length of AB = im

• Length of BC = iim

• Length of CD = iiim

• Length of DA = ivm

• The area of the given plot ABCD = ½ DB ( vm + vim ) [ Area of quadrilateral ]

= (……..) m2

Points to be noted……The Earth’s North and South poles act like a huge

magnet. One pole is positive and one pole is negative.Because magnetic and true North are not the same,

corrections are made in surveying to compensate for this difference. The difference is referred to as magnetic declination.

Cont…All magnetic objects, such as belts, watches, keys, and

other metal objects can interfere with the compass reading.

Hold compass away from metal objects while taking a reading.

The ability to use a compass effectively is an essential skill for wildlife managers, biologists, and other scientists who work outdoors.

Errors in Compass observations:The errors may be classified as  

• Instrumental errors

• Personal errors

• Errors due to natural causes

Instrumental errors• They are those which rise due to the faulty adjustments of

the instruments. They may be due to the following reasons:– The needle not being perfectly straight.– Pivot being bent– Sluggish needle– Blunt pivot point– Improper balancing weight– Plane of sight not being vertical– Line of sight not passing through the center of graduated

ring

Personal errors :They may be due to the following reasons:

Inaccurate leveling of the compass box.Inaccurate centering.Inaccurate bisection of signals.Carelessness in reading and recording.

Natural Errors:• They may be due to following reasons:– Variation in declination– Local attraction due to proximity of local attraction

forces. – Magnetic changes in the atmosphere due to clouds

and storms.– Irregular variations due to magnetic storms etc.

CONCLUSION :In this session we learnt :• Different parts of a compass and various accessories used

along with compass• Prismatic compass• Adjustments of a compass• Bearing• Plotting a given area using Intersection method of

compass surveying• Errors in compass observations

REFERENCES

C. Venkatramaiah, 1996, Textbook of Surveying, Pp 70-105

S.S. Bhavikatti, 2009, Surveying and Levelling, Volume 1, Pp 124-146.

en.wikipedia.org/wiki/survey/compasswww.freewebs.com/surveying/surveytyeps.htmengineeringsurvey.blogspot.com/2012/12/compass-

survey.

THANK YOU