Local Attraction problems in Surveying pdf

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local attraction problems in surveying pdf
local attraction problems in surveying pdf

WHAT IS LOCAL ATTRACTION ?

A magnetic meridian at a place is established by a magnetic needle which is uninfluenced by other attracting forces. However, sometimes, the magnetic needle may be attracted and prevented from indicating the true magnetic meridian when it is in proximity to certain magnetic substances. Local attraction is a term used to denote any influence, such as the above, which prevents the needle from pointing to the magnetic North in a given locality. 

Local Attraction problems in Surveying pdf available at bottom

Local attraction in compass surveying may exist due to are magnetite in the ground, wire carrying electric current, steel structures, railroad rails, underground iron pipes, keys, steel-bowed spectacles, metal buttons, axes, chains, steel tapes etc., which may be lying on the ground nearby. 

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Detection of Local Attraction. 

The local attraction at a particular place can be detected by observing the fore and back bearings of each line and finding its difference. If the difference between fore and back bearing is 180°, it may be taken that both the stations are free from local attraction, provided there are no observational and instrumental errors. If the difference is other than 180°, the fore bearing should be measured again to find out whether the discrepancy is due to avoidable attraction person, chains, tapes etc. It the difference still remains, the local attraction exists at one or both the stations.

Strictly speaking, the term local attraction in compass surveying may exist due to unavoidable attraction due to things about the person or to other sources not connected with the place where the needle is read. 

What is the correction for local attraction

If there is local attraction at a station, all the bearings measured at that place will be incorrect and the amount of error in all the bearings. There are two methods for eliminating the effects of local attraction.

First Method. 

In this method, the bearings of the lines are calculated on the basis of the bearing of that line which has a difference of 180 ° in its fore and back bearings. It is, however, assumed that there are no observational and other instrumental errors. The amount and direction of error due to local attraction at each of the affected station is found. If, however, there is no such line in which the two bearings differ by 180 °, the corrections should be made from the mean value of the bearing of that line in which there is least discrepancy between the back sight and fore sight readings. 

If the bearings are expressed in quadrantal system, the corrections must be applied in proper direction. In 1st and 3rd quadrants, the numerical value of bearings increase in clockwise direction while they increase in anti-clockwise direction in 2nd and 4th quadrants. Positive corrections are applied clockwise and negative corrections counter-clockwise. 

Second Method.

This is more a general method and is based on the fact that though the bearings measured at a station may be incorrect due to local attraction, the included angle calculated from the bearings will be correct since the amount of error is the same for all the bearings measured at the station. The included angles between the lines are calculated at all the stations. If the traverse is a closed one, the sum of the internal included angles must be (2n - 4) right angles. If there is any discrepancy in this, observational and instrumental errors also exist. Such error is distributed equally to all the angles. Proceeding now with the line, the bearings of which differ by 180°, the bearings of all other lines are calculated.

Special case : 

Special case of local attraction may arise when we find no line which has a difference of 180° in its fore and back bearings. In that case select the line in which the difference in its fore and back bearings is closest to 180°. The mean value of the bearing of that line is found by applying half the correction to both the fore and back bearings of that line, thus obtaining the modified fore and back bearings of that line differing exactly by 180°. Proceeding with the modified bearings of that line, corrected bearings of other lines are found.

Problems on local attraction in surveying

The following bearings were observed while traversing with a compass. 

local attraction in surveying pdf
local attraction in surveying pdf

Mention which stations were affected by local attraction and determine the corrected bearings. (U.B.)

Solution. 

On examining the observed bearings of the lines, it will be noticed that difference between back and fore bearings of the line DE is exactly 180°. Hence both stations D and E are free from local attraction and all other bearings measured at these stations are also correct. Thus, the observed bearing of DC (i.e 209° 10') is correct. The correct bearing of CD will, therefore, be 209° 10 - 180° = 29 10 while the observed bearing is 29 45'. The error at C is therefore + 35' and a correction 35' must be applied to all the bearings measured at C. The correct bearings of CB thus becomes 277° 5'-35' 276° 30' and that of BC as 276° 30' 180 ° = 96° 30'. The observed bearing of BC is 96° 55' Hence the error at B is + 25' and a correction of - 25' must be applied to all the bearings measured at B. The correct bearing of BA thus becomes 226° 10'- 25' 225° 45', and that of AB as 225° 45'-180 ° = 45° 45' which is the same as the observed one. Station A is, therefore, free from local attraction. The results may be tabulated as under

local attraction in compass surveying may exist due to
local attraction in compass surveying

Apply the corrections if the bearings of the previous example are measured in the quadrantal system as under : 

what is the correction for local attraction

Solution 

By inspection of the observed bearings, stations D and E are free from local attraction and hence bearings of ED, DE and DC are correct. The correct bearing of CD will, therefore, be N 29 ° 10' E. Since the observed bearing of CD is N 29 ° 45' E. the magnetic needle at C is deflected by 35' towards West. 

The corrected bearings of CB will, therefore, be N 82° 55' W + 35' = N 83° 30' W. The corrected bearing of BC will be S 83° 30' E. Since the observed bearing of BC is S 83° 05' E, the needle at B is deflected by 25' towards East. Hence the corrected bearing of BA will be S 46° 10' W - 25' = S 45° 45' W. The bearing of line AB will be N 45° 45' E, which is the same as the observed one. Station A is, therefore, not affected by local attraction.

The following are bearings taken on a closed compass traverse: 

problems on local attraction in surveying
problems on local attraction in surveying

Compute the interior angles and correct them for observational errors. Assuming the observed bearing of the line CD to be correct adjust the bearing the remaining sides. 

Solution.

ZA = Bearing of AE - Bearing of AB ZB = 130° 15'- 80° 10' = 50° 5'
B=Bearing of BA - Bearing of BC = 259° - 120° 20' = 138° 40 
ZC = Bearing of CB - Bearing of CD = 301° 50' - 170° 50' = 131° 0 
ZD = Bearing of DC - Bearing of DE 350° 50' - 230° 10' = 120° 40 
ZE = Bearing of ED - Bearing of EA =49° 30' - 310° 20'+ 0° = 99° 10' 
ZA + ZB + C + ZD + ZE = 50° 5' + 138° 40' + 131° 0' + 120° 40'+99° 10' = 539° 35' 

Theoretical sum = (2n – 4) 90 ° = 540 ° 

Error = 25'

Hence a correction of + 5' is applied to all the angles. The corrected angles are: ZA 50° 10'; ZB = 138° 45'; ZC=131° 5'; ZD = 120° 45' and ZE = 99 ° 15'

Starting with the corrected bearing of CD, all other bearings can be calculated as under: 

Bearing of DE = Bearing of DC- ZD 350° 50' - 120° 45 = 230° 5' . 
Bearing of ED = 230° 5' - 180 ° = 50° 5'
Bearing of EA = Bearing of ED - ZE = 50° 5' - 99° 15' + 360° = 310° 50' 
Bearing of AE = 310° 50' 180° = 130° 50' 
Bearing of AB = Bearing of AE-A = 130° 50'- 50° 10' 80° 40' 
Bearing of BA = 80° 40'+ 180° = 260° 40' 
Bearing of BC = Bearing of BA ZB = 260° 40' - 138° 45' = 121° 55' . 
Bearing of CB = 121° 55 + 180° = 301° 55' 
Bearing of CD = Bearing of CB - LC = 301° 55' 131° 5' = 170 ° 50' . 
Bearing of DC 170 ° 50' 180 ° 350 ° 50'. (Check)

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