surveying lecture 02.03.2013

7/27/2019 Surveying Lecture 02.03.2013

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COMPASS SURVEYING

It is that kind of survey in which compassis used for making angular measurementof a traverse. In compass surveying todetermine the direction of various surveylines of a traverse a compass is used tomeasure the angles which these linesmake with magnetic meridian. Howeverthe process of chaining and offsetting issame as in chain surveying.


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COMPASS SURVEYING

Suitability:When survey is to be conducted along along narrow strip like road, canal, railway.

When area can not be divided in totriangles.When surveying is to be conductedacross a jungle or in a dense forest.When area to be surveyed is hilly andchaining is difficult and liable to beerroneous.


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COMPASS SURVEYING

This kind of survey is not suitable forareas which is full of magnetic rocks oriron ores or having high voltage electric

lines.


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COMPASS

It is a small instrument that consists of amagnetic needle, a graduated ring and aline of sight. When the line of sight isdirected towards a particular line, themagnetic needle points towards themagnetic meridian and the anglebetween the line and the magneticmeridian is read on the graduated ring.


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COMPASS

TYPES:

There are two types of compass:

1. Prismatic Compass

2. Surveyor's Compass


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COMPASS

Prismatic Co

mpass:


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COMPASS

Working of Prismatic Compass: Centring

Levelling

Observing Bearings


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BEARING

Bearing of a Line:

It is the horizontal anglewhich a line makes withsome reference direction.


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MERIDIAN

It is that reference direction

from which bearing of anyline is observed.


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TYPES OF MERIDIANS

TRUE MERIDIAN

MAGNETIC MERIDIAN ARBITRARY MERIDIAN


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TYPES OF MERIDIANS

MAGNETIC MERIDIAN

It is the direction indicated bya freely suspended and properlybalanced magnetic needle

unaffected by the local attractiveforces.


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TYPES OF MERIDIANS

ARBITRARY OR ASSUMED MERIDIAN

For small surveys, it is anyconvenient assumed direction.


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TYPES OF BEARINGS

TRUE BEARING

MAGNETIC BEARING

ARBITRARY BEARING


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TYPES OF BEARINGS

TRUE BEARING

The angle which any line

makes with the true meridian istermed as true bearing.

It is also known as azimuth.


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TYPES OF BEARINGS

MAGNETIC BEARING:

The angle which any line makes withthe magnetic meridian is termed as truebearing or simply bearing.


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TYPES OF BEARINGS

ARBITRARY OR ASSUMED BEARING:

The angle which any linemakes with the arbitrary meridianis termed as true bearing or simply

bearing.


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DESIGNATION OF BEARINGS

The bearings are expressed in followingtwo ways:

1. Whole Circle Bearings

2. Quadrantal Bearings


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Whole Circle Bearings:It is the horizontal angle whicha line makes with the North

direction of the meridian measuredin clockwise direction having valuesvarying upto 360 o. A prismaticcompass is used to measure thistype of bearings.


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QUADRANTAL BEARINGS:It is the horizontal angle that a line

makes with either North or Southdirection of the meridian whichever isnearer measured in clockwise oranticlockwise directions towards east orwest of meridian having numerical valuevarying upto 90 o. A surveyors compass isused to measure this type of bearing.


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REDUCED BEARINGS (R. B.):The value of W.C.B. observed when

exceeds 90 o is converted into

corresponding bearings in the quadrantalsystem of bearings, which has samenumerical values of the trigonometricalfunctions. These converted bearings aretermed as reduced bearings.


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REDUCED BEARINGS (R. B.):

Conversion of W. C. B. into corresponding R. B.


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DESIGNATION OF BEARINGSFORE AND BACK BEARING:

The bearing of a line observed in thedirection of progress of survey is known as forebearing (F. B.) of the line; while observed in the

opposite direction of the progress of survey istermed as back bearing (B. B.) of the line.Numerically, in WCB system the two bearings of the line differ by 180 o and in quadrantal systemit is same but having opposite cardinal points.

Mathematically: FB = BB 180 o


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LOCAL ATTRAACTIONSometimes due to the presence of magnetic substances or electric current,the magnetic needle of the compass doesnot points towards the actual magneticnorth or south. Therefore, it is said to beunder the influence of external attractiveforces. Such disturbing forces whichprevents the magnetic needle to occupyits normal position are named as localattractive forces.


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LOCAL ATTRAACTION

CAUSES:

Natural Causes like proximity to magneticrocks or iron ore.

Artificial causes like presence of steel

structures, railway lines, iron pies, currentcarrying wires, steel buttons, bunch of keys, use of iron ranging rods etc.


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LOCAL ATTRACTION

DETECTION:

Local attraction at any particular place canbe detected by observing both fore andback bearing of the survey line. If thedifference of the two differ by 180 o then

there is no local attraction and if not thenexist local attractive forces and either of the two will be affected by local attractiveforces.


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MAGNETIC DECLINATION

Barring a few places, the magneticmeridian does not coincide withthe true meridian of that place.

Therefore, the horizontal anglewith which the magnetic meridianfails to coincide with true meridianat that particular place is termed asmagnetic declination of that

particular place.


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If the magnetic needle is towardseast of the true meridian, the

magnetic declination is said to beeast declination and if it towardswest then the declination is termed

as west declination.


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CALCULATION OF BEARINGS:

True Bearing = Magnetic Bearing Magnetic Declination

Use +ve sign when magnetic declination is towards eastotherwise use ve sign.


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MAGNETIC DIPThe amount of magnetic dip varies from

place to place on the earth surface. Itdepends upon the distance from the equator.Magnetic dip of the needle is zero at theequator and maximum 90 o at the poles.

To keep the magnetic needle in thehorizontal position a brass weight, rider, is

used at a suitable point over the higher end of the needle.


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THANK YOU

surveying lecture 02.03.2013

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