pract 1b, correcting for la -2102, 2103,2105-final

Surveying Laboratory, Civil Engineering Deptt., CWIT Pune – 1. Pract No. 1B Date Ti me __ am/p m to _ am/p m Place CWIT, Pune 1 COMPASS SURVEYING Object: Finding fore and bac bearing of line! of triangle appl"ing correction for local attraction. Instrument: Pri!matice compa!!, ranging rod #$ number, c%al piece! etc. Theory: T%e magnetic needle doe! not point to t%e magnetic nort% & !eriou!l" deflected from it! normal po!ition in 'icinit" of iron ob(ect, po)er line, magnetic roc! etc. *uc% t% ing a! )ell ang ul ar de 'i ation of magneti c needle! in %ori+ontal plane from it ! normal po!ition due to !uc% t%ing i! called a! local attraction. ocal attraction ma" include an" !ource of attraction. *uc% a! bunc% of e"!, nife, !teel -eamed !pectacle!, )ri !t)atc %, c%ai n rai l!, arro )!, cleari ng ae, !te el !tructure, electric/p%one cable!, iron pipe!, railing!, iron lamppo!t, automobile, lectronic, gadget! lie cell p%one, pager!, calculator!, electronic dairie! etc. in t%e 'icinit" of compa!!. 0t a gi'en place local attraction ma" c%ange from !tation and time to time. T%e local attraction i! !ome for eac% of barring ob!er'ed at affected !tation!, %ence difference of bearing of t%e line at t%e !tation! )ill gi'e correct 'alue of angle e'en T%oug%t t%e !tation i! !ub(ect to t%e local attraction. To detect t%e local attraction ob!er'ed FB & BB of a line, if t%o!e differ! b" 123 t%en eit%er bot% of terminal !tation! are free from local attraction or bot% of t%em are !ub(ected to e4ual local attraction, pro'ided t%ere i! no in!trumental error. If local attraction at t)o con!ecuti'e !tation! are different t%e FB & BB of a line ob!er'ed from t%o!e !tation! )ill not differ b" 123, t%o!e bearing need! to be corrected for local attraction. Procedure: Prt I: Mesur!n" #$ % $$ o& '!nes o& Tr(erse 5ar a tra'er!e on ground b" naming it! 'ertice! 0, B, C, 6. in anticloc)i!e direction. 0t !tn. 0 mea!ure FB of 0B and BB of C0, at !tation B mea!ure FB of BC and BB of 0B at !tn. C mea!ure FB of C0 and BB of BC. T%i! )a" go on mea!uring bearing! at all re!t of t%e tra'er!e !tation!. Prt II: Correct!on &or 'oc) Attrct!on 1. T%e proce dure of cor recti ng bearin g i! al) a"! !tart ed from B. B. of an u naffec ted lin e. $. If FB & BB of no line differ b" 1 23, t%en bearing of a l ine )%o!e d iffer ence of FB & BB i! near to 123 are ad(u!ted to differ b" 123. If difference of FB &BB for !uc% a line i! le!! t%an 123 !a" 1783, t%en add %alf t%e di!crepanc" i.e. 9123 # 1783:/$ ; <2= to eit %er FB or BB )%ic% i! %ig%er and !ubtract !ame %alf di!crepanc" of <2= from eit%er FB or BB )%ic% i! lo)er. If difference of FB & BB !uc% a line i! more t %an 12 3 !a" 123<2= t%en Practical Write ups in Surveying for PTD E CIvil Page 1

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Page 1: Pract 1B, Correcting for LA -2102, 2103,2105-Final

8/11/2019 Pract 1B, Correcting for LA -2102, 2103,2105-Final

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Page 2: Pract 1B, Correcting for LA -2102, 2103,2105-Final

8/11/2019 Pract 1B, Correcting for LA -2102, 2103,2105-Final

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

!ubtract %alf t%e di!crepanc" i.e. 91 23<2=#1 23:/$ ; 1>=from eit%er FB or BB )%%ig%er & add !ame %alf di!cripanc" of 1>= to eit%er FB ?- BB )%ic% i! lo)er.

<. Calculation of interairl" included angle!Interiarl" included angle bet)een t)o line at !tation! i! difference of bearing of t%o!e t)oline a! of ob!er'ed from t%at !tation. If )e %a'e to find out interiarl" included ang

bet)een line 0B & BC at !tation!. B, t%en )e %a'e to ta e difference of bearing 0B &BC a! ob!er'ed from !tation B. From !tation! B, )e )ill ob!er'ed BB of 0B & FB of BC, %ence angle 0BC )ill be difference of BB of 0B & FB of BC. If !uc% differencmore t%an 1 23, t%en 'alue calculated ma" of t%e Corre!ponding e teriarl" incluangle. T%i! )a" calculate interiarl" included 0ngle at all !tation of clo!e tra'er!e "ou%a'e condected . Wor out t%erotical !um of included angle u!ing relation @A$N #823E, )%ere N i! number if !ide of clo!e tra'er!e .!ub!tract t%erotical !um of includangle from t%ere actual !um To find out t%ere total angular error .If t%i! total angulain t%e permi!!ible imit A C N , t%i! di!tribute it among!t all t%e angle in multiple!minute! onl" , *uc% t%at t%eir actual !um ; t%eoretical !um .

. Calculating bearing! corrected for local attraction commencing from BB of unaffec

line & u!ing corrected included angle t%e correct bearing of !ucce!!i'e line are calculau!ing formulaFB of ine BB of line merging included anglemerging ; into t%at !tation G bet)een t%o!eFrom !tation t)o line!Plea!e note t%at bearing of line can not be more t%an <H23. If calculated Bearing ofmore t%an <H2 3 t%en !ubtract <H23 from it

F.B of a line BC BB of a line 0B included angle 0BCmerging/!tarting ; merging/ending G i.e. angle bet)een

From !tn. B. in !tn. B line 0B & BC. Re&erence:For tabulated ob!er'ed & corrected bearing! refer table on page No. FB ___________

Practical Write ups in Surveying for PTD E CIvil Page $