Surveying Laboratory, Civil Engineering Deptt., CWIT Pune 1.

Surveying Laboratory, Civil Engineering Deptt., CWIT Pune 1.

Pract No. 1ADate

Time______ am/pm to _____ am/pm

PlaceCWIT, Pune 1

COMPASS SURVEYING

Object:Study of prismatic compass.Instruments:Prismatic compass, Ranging rods-2 nos., chalks pieces etc.

Theory:Principal operation in surveying is to determine relative positions of the ground stn. w.r.t. each other in horizontal and vertical planes. To determine positions of a point w.r.t. other point in horizontal plane, we must know distance between them and direction of a line joining them.

The direction of a line joining two station may be determine in terms of an angle made by that line either with its adjoining line or with a reference direction. The reference direction used in measurement of direction Of direction of the lines is called as meridian and the Angle made by a line under consideration with the Reference direction i.e. meridian is called as Bearing of a line.

There are three types of meridian viz. True meridian, arbitrary meridian and magnetic meridian. The direction shown by freely suspended or freely supported magnetic needle is called as magnetic meridian. The bearing of a line measured w.r.t. Magnetic meridian is called as magnetic bearing of that line. The magnetic bearing of a surveyors compass or a prismatic compass.

Prismatic Compass:The direction measuring or indicating instrument fitted with magnetic needle is called as compass. The compass fitted with a prism to facilitate reading procedure is called as prismatic compass.

The prismatic compass is a simple and portable instrument used to measure direction of line. It consists of broad from magnetic needle centrally pivoted in a non-magnetic metal box, with help of hardened steel pivot & agate cap. The needle caries light aluminum ring graduated up to 1/20 in clockwise direction. The zero on this ring is towards south end of needle so that instrument will read zero when north is bisected. To dampen oscillation of the provided offspring break diametrically opposite are fixed to the box the object Vane with horse hair and the reflecting Prism with eye slit. The line passing through horse hair and eye silt is a line of sight for storage/transportation the Prism may be folded against the box of the compass and held in position with folding strap. The object when can also be folded but against the cover glass. When folded against the cover glass, the object vane operates lifting lever, which lifts needle off the pivot point and thus avoids its blunting especially during storage/transport. The object vane is fitted with a mirror to bisect too high or too low objects. To facilitate bisection or bright objects like sun, the colored sunglasses are also provided.

Fig: Cross Sectional View of a Prismatic Compass

Temporary Adjustment Of A Prismatic Compass:

The adjustment required to be performed at every instrumental set up in to make the instrument ready for use are called as its temporary adjustments. There are three temporary adjustments for a prismatic compass.

i) Setting up of tripod, taking out instrument from case and fixing it on tripod head- set up the tripod in such a way that its height is suitable to your height, its legs are spread well apart and if the ground is sloping, two legs are on falling side and third one on rising side. Then take carefully out the compass from its case, remove its cover, put it in a case and fix the compass on tripod head by rotating fixing sleeve/ collar and not by rotating the compass.

ii) Centering; centering is process to bring the center of instrument exactly on same vertical line passing through ground stn. Centering is done by shifting the leg of instrument till a pebble dropped from bottom of instrument drops exactly on the ground stn. mark.

iii) Leveling of a compass is done by eye judgment and with help of a ball and socket arrangement at tripod head till the aluminum-graduated ring freely rotates in near horizontal plane.

Reading Magnatic Bearing By A Prismatic Compass:

Let we want to measure bearing of line AB with the prismatic compass. Set up compass at A, unfold its object vane and sighting prism and perform all its temporary adjustments. Then direct object vane of a compass towards stn. B and bisect it. Ascertain that cap of prism is Removed and now read through prism. The Graduated ring is read from right to left i.e. From lower reading to higher reading, exactly Up to 30 and by estimation up to 10 In adjoining sketch the compass is showing Reading of 114030.

.Fig: Magnetic Bearing of a line .

Fig: Reading Magnetic Bearing with a Prismatic CompassConcept Of Fore Bearings (FB) And Back Bearing (BB):The bearing of a line measured in forward direction of traversing or measured from its starting terminal stn. is called as its fore bearing (F.B.) where as the bearing of the same line measured in backward direction of traversing or measured from its end terminal stn. is called as its back bearing (B.B.) i.e. bearing of line AB measured from stn. A is called as fore bearing. Where as that measured from B is called as its back bearing. If ther is no local attraction i.e. local influencing force such as iron objects, power lines, magnetic rocks etc. in the vicinity of the terminal stn. the F.B. and B.B. of a line differs exactly through 180. If F.B. and B.B. of a line do not differs by 1800.then either one or both of the terminal stns. Are subjected to local attraction and if they differ exactly then either both of the terminal stns. are free local attraction or are subjected to equal local attraction.

Practical Write ups in Surveying for PTD FE CIvilPage 1