COMPASS SURVEYING

In Compass survey chain or tape is used for linearmeasurements and compass is used for fixingdirection. In compass freely suspended magnetic needle directsto north- south and the bearing of line is obtained byline of sight. When large area are involved, compass surveying isused. There are mainly two methods of land measurement.1) Triangulation survey2) Traverse survey

INTRODUCTIONINTRODUCTION

Traversing is that type of survey in which anumber of connecting survey lines form theframe work and the directions and lengths ofthe survey lines are measured with the help ofan angle measuring instrument and a taperespectively. A traverse may be either Open orClosed as shown below.

Closed traverse Open traverseSt. ASt. E St. B

St. D St. C

St. A

St. E

St. BSt. DSt. C

An Open traverse begins at a point of knownposition and ends at a station whose relativeposition is unknown. This type of traverse isfrequently used for preliminary surveys forhighways.

Open traverseSt. A

St. E

St. BSt. DSt. C

6015

A closed traverse begins and ends at the samepoint whose position is known. An example ofthis type of traverse is a perimeter survey of alake or pond.

Closed traverseSt. A

St. E St. B

St. D St. C

A compass is a small instrument which mainlyconsists a magnetic needle, a graduated circle and aline of sight. The principle of magnetic compass is, If a long, narrow strip of steel or iron is magnetized,

and is suitably suspended or pivoted about a pointnear its centre, it will oscillate freely about the verticalaxis and will tend to establish itself in the magneticmeridian

There are mainly three types of magnetic compass,1) Prismatic compass2) Surveyors compass

COMPASSCOMPASS

The magnetic compass never gives a anglebetween two survey lines directly, it gives onlythe angle of survey lines with reference tomagnetic meridian at particular station. So, to determine the angle between two surveylines, person has to do manually calculation onrecorded bearings.

The prismatic compass is the most convenient andportable form of magnetic compass which can either beused as a hand instrument or can be fitted on a tripod. The greatest advantage of prismatic compass is thatboth sighting the object as well as reading circle can bedone simultaneously without changing the position ofthe eye. Prismatic Compass comprises of a magnetic needleattached to the circular ring made up of aluminum. The needle is on the pivot and will orient itself in themagnetic meridian, Therefore the north and south endsof the ring will be in this direction. The line of sight is defined by the object vane and theeye slit, both attached to the compass box.

THE PRISMATIC COMPASSTHE PRISMATIC COMPASS

The object vane consist of a vertical hair attached to asuitable frame while the eye slit consist of a vertical slit cutin to the upper assembly of the prism unit, both beinghinged to the box. A triangular prism is fitted below the eye slit, havingsuitable arrangement for focusing to suit different eyesight. The readings increase in clockwise direction from 0o atSouth end 90o at West end 180o at North end and 270o atEast end. The object vane frame can be folded on the glass lid whichcovers the top at box. The object vane presses against a lifting lever which liftsthe needle of the pivot and holds it against the glass lid. When bright objects are sighted dark glass may beinterposed in to the line of sight.

PARTS OF PRISMATIC COMPASS1. Compass Box 2. MagneticNeedle3. Graduated Ring 4. Pivot5. Objective Vane 6. Eye Vane7. Adjustable Mirror 8. Spring Brake9. Brake Pin 10. Lifting Lever11. Lifting Pin 12. Prism13. Focusing Stud 14. Glass Cover15. Prism Dust Cap 16. Sun Glasses

Surveyor compass acquires its name from itsextensive use by surveyors. But the prismaticcompass has now replaced as it is light, compact, andhandy. It is similar in construction to the prismaticcompass except for a few differences as follows: The graduated ring is attached to the circular box andnot to the magnetic needle. The eye vane consists of metal vane with a fine sighthole. As the compass box is turned, the letters N, E, S, andW turn with it, but the needle continues to pointtowards the north and gives a reading which isdependant on the position of the graduated circle.

THE SURVEYORS COMPASSTHE SURVEYORS COMPASS

The 0o is placed at both north and southdirections and 90o is marked at east and westdirections. Normally, surveyors compass are availablewith inbuilt level tube. So, no external leveltube is required for levelling purpose. It gives the reading of survey line in quadrantbearing system.

TEMPORARY ADJUSTMENT OFA PRISMATIC COMPASS

Before starting a compass survey, the compassshould be adjusted properly for accuratereading. The following are the adjustmentsusually necessary in the prismatic compass:1) Centering2) Leveling3) Focusing4) Observing the bearing of a line

1) CENTERING: The center of the compass is placed verticallyover the station point by dropping a small pieceof stone below the center of the compass, it fallson the top of the peg marking that station.

2) LEVELLING: By means of ball and socket arrangement theCompass is then leveled the graduated ringswings quite freely. It may be tested by rolling around pencil on the compass box.

3) FOCUSSING : The prism attachment is slide up or downfocusing till the readings are seen to be sharpand clear.

4) OBSERVING BEARING: The compass centered over station A of the lineAB and is leveled. Having turned vertically the prism and sightingvane, raise or lower the prism until thegraduations on the rings are clear and lookthrough the prism. Turn the compass box until the ranging rod atthe station B is bisected by hair when lookedthrough the prism. When ranging rod B comes in line with the slit ofeye vane and the vertical hair of the object vane,the reading is taken through the prism, whichgives the bearing of line AB.

The horizontal angle made by any survey linewith respect to some reference line is calledBearing. The bearing of a line is always measured inclockwise direction with respect to somereference line or direction. This fixed referenceline is known asMeridian.i. True meridian & True bearing The line passing through the true north andsouth pole is called True meridian, and thehorizontal angle which the line makes with thetrue meridian is called True bearing.

MERIDIAN & BEARINGMERIDIAN & BEARING

ii. Magnetic meridian & magnetic bearing The direction shown by a freely suspendedneedle which is magnetized and balancedproperly without influenced by any other factorsis known asmagnetic meridian. The horizontal angle which the line makes withthe magnetic meridian is calledMagnetic bearing.iii.Arbitrary meridian & arbitrary bearing In case of small works or in places where truemeridian or magnetic meridian can not bedetermine, then any direction of a prominentobject is taken as a reference direction called asArbitrary meridian. The horizontal angle which the line makes withthe arbitrary meridian is called Arbitrary bearing.

iv.Grid Bearing The grid bearing of a line is the horizontalangle which the line makes with the gridmeridian. The Grid north is generally used with GPS.

INSTRUMENTS USED FORCOMPASS SURVEYINGThe various instruments used in the compasssurvey are :

Prismatic compass chain Tape Ranging rods Tripod Plumb-bob Arrows

There are two systems commonly used to expressthe bearing.WHOLE CIRCLE BEARING (W.C.B.)In this system the bearing of a line measured withthe magnetic north in clockwise direction. Thevalue of bearing thus varies from 0o to 360o.QUADRANTAL SYSTEM (Q.B.)In this system the bearing of a line is measuredeastward or westward from north or southwhichever is near. The directions can be eitherclock wise or anti clockwise depending upon theposition of the line.

BEARING SYSTEMSBEARING SYSTEMS

REDUCED BEARING (R.B.) When the whole circle bearing of a line isconverted into quadrantal bearing, its calledReduced Bearing, which varies between 00to900 and it should be mentioned with properdesignation.

Every line has two bearings which observed ateach end of the line. The bearing of the line in the direction ofprogress of the survey is called Fore Bearing(FB), while the bearing in the oppositedirection is called Back Bearing (BB). Therefore BB of a line differs from FB byexactly 180o.

FORE BEARING & BACK BEARINGFORE BEARING & BACK BEARING

The angle between true north and magnetic north iscall the declination or variation. This angle can be computed approximately from thelocation of the North magnetic pole. Declination values represent the error betweenmagnetic North and True North, for a specific location. Declination degree values can be either expressed asEasterly and Westerly, or as positive and negativevalues. Easterly declinations are represented as positive (+)values. Westerly declinations are represented as negative (-)values.

MAGNETIC DECLINATIONMAGNETIC DECLINATION

Lines shown on the map through the placesthat have the same declination are calledIsogonic line.

Line drawn through places that have zerodeclination is called an Agonic line.

Variations in declination Declination at any point does not remain constant butkeeps on changing from place to place & time to time.There variations may classified as under,a) Diurnal variationb) Annual variationc) Secular variationd) Irregular variation Depending on where you are, the angle between truenorth and magnetic north is different. In the U.S., theangle of declination varies from about 20 degrees westin Maine to about 21 degrees east in Washington. Themagnetic field lines of the earth are constantlychanging, moving slowly westward ( to 1 degreeevery five years).

The errors may be classified as,Instrumental errorsPersonal errorsErrors due to natural causes

ERRORS IN COMPASS OBSERVATIONSERRORS IN COMPASS OBSERVATIONS

The magnetic needle always point towards themagnetic meridian when its freely suspended andtruly balanced on a pivot. But, when the compass is set up on the field in thepresence of iron or steel structure, or any highvoltage current, then the magnetic needle will notpoints towards the magnetic north but it willdeflected from its normal position. This deflectionof the needle from the magnetic north under theinfluence of magnetic substances is known asLocal attraction. So, when the survey station are affected by thelocal attraction, the measured bearing should becorrected before plotting the traverse.

LOCAL ATTRACTIONLOCAL ATTRACTION

Sources of Local attractionThe needle of the magnetic compass may beaffected by following sources,a) Natural sources

Iron ores Magnetic rocks

b) Artificial sources Steel structures Rails Iron pipes Electric lines Metallic chain Steel tapes

ELIMINATION OF LOCAL ATTRACTION1) First method In this method, first of all interior angles of atraverse are calculated from the observedbearing. Then the angular check is applied tothe calculated angles. If the sum of theinterior angles is not equals to (2N-4) X 90,then the total error is equally distributedamong all the angles of the traverse. Then starting from the unaffected line, thebearing of all the lines may be corrected byusing the corrected angles.

2) Second method In this method, unaffected line is firstly detectedfrom the given reading. Then starting from thatline, the bearing of other affected lines arecorrected by finding out the amount of error ateach stations.If no lines is differ by 1800 in its fore and backbearing, then select a line in which thedifference of its FB and BB is nearer to 1800.then find out the mean value of the bearing ofthat line by applying half the correction to boththe fore and back bearing of that line.The modified bearing will be differ by 1800, soproceed with the modified bearing to find outcorrected bearing of remaining lines.

CHAIN AND COMPASS SURVEYING FIELD WORKI. ReconnaissanceII. Preparation of index sketchIII. Marking the station on the groundIV. Locating stationsV. Measuring and recording of survey linesVI. Measurement of bearing of survey lines