- see E-27 COMPASS SURVEYING _xPeRIMENT- 2 () (a), (b), (c) 1. Study of Prismatic compass 2. Setting the compass and taking observations. 3, Measuring angles between the lines meeting at a point. Apparatus: Prismatic compass with stand, ranging rods, rolling pencil, black thread | obeused as bisecting hair), wooden Pegs, field book ete, P Theory. When it is not possible-to-devide the area into network of triangles as is done inchain surveying, we adopt the method of compass surveying. The method employed js traversing, in which a number of connected lines are run in the area to be surveyed whose lengths are measured with chain on tape as usual and directions are determined with prismatic compass. ‘Thus in traversing when compass is used for making angular measurements, itis known‘as compass traversing or compass surveying. In this surveying the compass is used to determine the direction of survey lines of the frame-work of the traverse by measuring the angles which these lines make with magnetic meridian. 1, Study. A compass may be defined as small instrument which consists of a magnetic needle, a graduated circle and a line of sight. When the line of sight is directed towards aline, the magnetic needle points towards magnetic meridian and the angle which the line makes with the magnetic meridian is read at the graduated circle. Prismatic Compass. It is very valuable instrument which is commonly used for rough surveys. It was invented by Captain Kater of Germany in 1814. It consists of following parts : (a) Compass metal box. Itis the main part which contains the whole compass which is about 10 cm dia. 1 ( Pivot. It is fixed in the centre of metal box which carries the magnetic needle. () Magnetic Needle. It is the part of compass which indicates the direction of North line. It may be edge type or broad type needle. (@ Graduated Ring. The main needle is attached to a ring which is made of aluminium. This ring is graduated to half a degree and is known as graduated ring, The graduation of this ring are from 0 degree to 360 degree. (@) Agate Cap. The cap serves the purpose of attachment of magnetic needle with the graduated ring. () Glass Cover. The cover protects the needle and other parts of compass from Weathering affects. It also helps in centring and levelling of the compass. The Compass is levelling by rolling pencil on the glass cover. 4 * ©) Prism. The reflecting prism is used to read the graduated ring to observe the onjects 3 to protect it from dust and moisture. ). A cap is provided on the prism : object properly generally a ranging Sighting Suit. This slit is used to view theE-28 EXPERIMENTS tod, through object vane and eye is placed at the eye hole below this slit. (j) Coloured Glasses. A pair of coloured glasses are provided, which are kept between the slit and the object vane when Sun or some other luminous object is to be bisected. (&) Focussing Screw . This screw is used to make the graduations of ring clearly visible which adjusts the prism to the eye sight. () Object Vane. This is placed diametrically opposite to the prism. This vane is used to sight the object. (m) House Hair. It is'a small hair or thread kept in object vane which is used for bisecting the object generally a ranging rod. This should be fine as possible, (n) Reflecting Mirror. This mirror which is movable is attached to the object vane, It is used to observe the bearings of very high or low objects. (0) Brake Pin. It is a pin which is used for pressing the spring brake. Whenever a reading is taken, this pin must be pressed to bring the needle stationary. (p) Spring Brake. It is a small light metallic strip which is attached to the inside of metal box. When brake pin is pressed, this strip comes in contact with the edge of ting which ultimately damps the oscillations of needle. (q) Lifting Pin. Itis a small pin which is attached to the compass box and slightly edging upward above the hinge of object vane. When object vane is brought _ down on the face of glass cover, this pin lifts the lever. ©) Lifting Lever. To prevent undue wear of pivot point, this lever is used to lift the needle automatically off the pivot point with the help of lifting pin. 2. Setting the compass and taking observations. Prismatic compass can be used while holding it in hand but generally it is mounted on a tripod stand which carries a vertical spindle in the ball and socket arrangement to which the compass is screwed. Setting of the compass involves the following operations : (a) Centering. The method of placing the centre of compass exactly over the station point is termed as centering. It is done simply by dropping a piece of small stone below the centre of the compass. When it fall at the top of the wooden peg marked as the station point, centering is assumed to be complete. (6) Levelling. After centering levelling of the compass has to be done to make the graduated ring swing freely. It is levelled by ball and socket arrangement on the compass stand. If it is not provided, levelling can be done by rolling a round pencil on the glass cover of compass box. (c) Observation of the bearing. After centering and levelling, observation of bearing of line is taken, When graduated ring swings freely needle will point towards North South direction. If bearing of OA line is to be observed, place a ranging rod at A. Prismatic compass is fixed at O. After initial observations i.e. centering and levelling compass box is tumed until the ranging rod at A is bisected, Use the brake-pin and bring the graduated ring at next and then take the reading at which the hair line appears to cut the graduated ring. Although the least count of compass is 30 minutes but readings are estimated upto nearest 15 minutes.pees E-29 ° 3, Measuring angles between the lines meeting at a point : The angle between the measured directly by the prismatic compass. To firstly their angles with the magnetic meridian values is found which is equal to the angle le COD is to be measured between the lines OC> expERIMENTS E-31 EXPERIMENT - 2 (it) Objects. Traversing with a prismatic compass and chain a closed traverse (Recording & Plotting by included angles). Apparatus. Prismatic compass with stand chain (standard), Arrows, ranging rods, measuring tape, flags, field book, wooden pegs etc. ‘Theory. A traverse may be defined as a series of connected survey lines of known lengths and directions. When a complete circuit is made by traversing, it is termed as closed traverse. A closed traverse is suitable for survey of moderately, large areas such as small towns and big villages and for locating boundaries of forests, lakes, streams etc. The process of traversing is done by a standard chain while prismatic compass is used for measuring the directions. Procedure : 1. First of all reconaisance (preliminary survey without any instrument) is performed and rough sketch is prepared. N N 2. Flag stations or station points are marked by D Cc fixing pegs and ranging rods with flags. These station points are so selected that they are all visible from one another and are close to the boundary of the field as possible. The location of these station points is then taken. w . The compass is set up at starting point (A) and bearings toward the back and forward stations are taken and recorded. The length of first line say AB is chained and offsets on both the sides of the chain line are taken similarly as in chain surveying. 4. When we reach at the forward station, length of first line (say AB) is recorded in our field book. 5. Now the compass is set at this second station (B) and forward and backward bearings are measured i.e. BC and BA. . Similarly BC is chained as offsets are taken on both sides and its length is recorded at station C. . Proceeding in this way the whole traverse is completed. For rough works, the prismatic compass can also be set up at alternative stations. This traverse may be Tun in clockwise or anti-clockwise direction. 8. Ifany important object is out of reach of an offset, it is taken by radiation method in our field work. For this bearing of this object is observed and length of radial line from the position of prismatic compass upto the object is measured and with these observations, it can be easily plotted in our map. . B a xEXPERIMENTS ee ey Check on closed traverse (Field check). Fore bearing and back bearings of a ling differ exactly by 180°. So if fore bearing of AB (measured from station A) and bearin of BA (observed from station B) which is back bearing of AB differ by 180°, the Work is correct and no errors (Maximum permissible erro is 15.) If there is error more than this limit, again take the reading. If again the result is same then there is a local attraction whichis to be corrected by finding a particular station which is free from it (minimum t bearings of other respective stations error) and proceeding from it to find out the correc! to remove the error. , Plotting of traverse (closed) (a) Firstly draw the meridian at th and plot the bearing of line 4B @, Measure the length AB to scale (as recorded in field book).Then at B with the help of a protractor (D) draw angle ABC (Z,) and cut off length BC to scale. Repeat the process at each succeding stations C, D etc. until the traverse is complete. There is-a liability of small error to be crept in *: because of measuring the angle with protactor in minutes, which is to be done by judgement...) + If the starting and ending points do not coincide in close traverse,there is an error of closure ofclosing error. ess dala _ This error has to be adjusted by graphical method. Figure /‘Sketch : deine is done by method of included angles. D e stating station A )as shown in Fig.