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Traverse Survey (Part 2)

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  Essential features of magnetic compass:General principle of magnetic compass: If a long, narrow strip of steel or iron is magnetized, and is suitably suspended or pivoted about a point near its center so that it can oscillate freely about the vertical axis, it will tend toestablish itself in the magnetic meridian at the place of observation. (a) Magnetic needle:   To establish the magnetic meridian. (b) A line of sight:  To sight the other end of the line. (c) A graduated circle:  Either attached to the box or to the needle to read the directions of the lines. (d) A house box (or compass box):  To house the above parts. Magnetic Declination:   The horizontal angle between magnetic meridian and true meridian shown by theneedle at the time of observation is known as declination.   If the magnetic meridian is to the right side of the true meridian, declination is saidto be eastern or +ve   If the magnetic meridian is to the left side of the true meridian, declination is said to be western or ve Determination of true bearing: If the magnetic declination at a place at the time of observation is known, the true bearing can be calculated from the observed magnetic bearing by following relation!True earing # $agnetic earing % &eclination'use + sign if the declination is to the east and  sign if the declination is to the west.( ***This rule is valid onl for !hole ircle #earing $stem%***&f reduced bearing is used' it is alas advisable to dra the diagram andcalculate bearing%Example: %%e  The magnetic bearing of a line ) is *  - -/ E. 0alculate the true bearing if the declination is 1 - -/ 2est. +ocal attraction (pg,-):  3ocal attraction is a term used to denote any influence 'such as,magnetic needle may be attracted and prevented from indicating the true magnetic meridian when it is in close proximity to certain magnetic substances.( which prevents the needle from pointing to the magnetic .orth  in a given locality. The magnetic needle will be deviatedfrom the magnetic meridian  due to the influence of magnetic substances. This phenomenonis known as local attraction. This is caused by the proximity of iron, steel structure, rails, 1  electric post etc. some other sources! magnetite in the ground, wire carrying electric current,underground iron pipes. )4& metal buttons, axes, chains, steel tapes etc. which may be lyingon the ground nearby. Example: %%f   The following bearings were observed while traversing with a compass.3ine5. . . .)  45 0 45’ 226 0 10’ 0  96 0 55’ 277 0 5’ 0&  29 0 45’ 209 0 10’ &E  324 0 48’ 144 0 48’ $ention which stations were affected by local attraction and determine the corrected bearings. $olution: 3ine6bserved earingError0orrection0orrected earing7emarks)  45 0 45’ - at )- at )  45 0 45’StationsB & Careafected by localattraction. )  226 0 10’ +8/ at 8/ at  225 0 45’ 0  96 0 55’ +8/ at 8/ at  96 0 30’ 0  277 0 5’ +8/ at 08/ at 0  276 0 30’ 0&  29 0 45’ +8/ at 08/ at 0  29 0 10’ &0  209 0 10’ - at &- at &  209 0 10’ &E  324 0 48’ - at &- at &  324 0 48’ E&  144 0 48’ - at E- at E  144 0 48’ Example: %%g  The following are bearings taken on a closed compass traverse!3ine5. . . .)  80 0 10’ 259 0 0’ 0  120 0 20’ 310 0 50’ 0&  170 0 50’ 350 0 50’ 2  &E  230 0 10’ 49 0 30’ E)  310 0 20’ 130 0 15’ 0ompute the interior angles and correct them for observational errors. )ssuming the observed bearing of the line 0& to be correct ad9ust the bearing of the remaining sides. $olution:+atitude:  The distance measured parallel to the magnetic meridian '4* line(.  :ositive when measured northward from srcin.  :ositive latitude termed as 4orthing.   4egative when measured southward from srcin.   4egative latitude termed as *outhing.  +atitude / +ength of the side * osine of its bearing A#   / A# * os 0Departure:  The distance measured parallel to the line which is perpendicular to the magneticmeridian 'E2 line(.  $ay be positive or negative.  :ositive departure termed as Easting.   4egative departure termed as 2esting.  Departure / +ength of the side * $ine of its bearing (reduced bearing) A#   / A# * $in 0 onsecutive o,ordinates:  The latitude and departure of any station with reference to the proceeding station are termed as consecutive 0oordinates of the station. &ndependent o,ordinates:  The latitude and departure of any station with respect to acommon srcin are termed as Independent 0oordinates. haracteristics of a losed Traverse:  *um of included angles # 'n  ;(< =- where n# number of sides.  *um of excluded angles # 'n+;(< =-.  &ifference between 5. . and . . is e>ual to ?- - .  *um of 4orthings # *um of *outhings.  *um of Eastings # *um of 2estings. 3   &ifference between the bearings of two lines at a station is the included or excludedangles. 4
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