Saturday, September 8, 2012

Local Attraction - Compass Surveying

Local Attraction: 
A compass shows the direction of the magnetic meridian on the principle of magnetism. Any magnet attracting  material, when is brought near to the compass needle, needle will deflect from the true magnetic north.

In that case, you will not read the true north direction and if you take the bearings of the lines in such condition there comes a error in the readings and that error is known as the local attraction.

Materials which are most likely to be present there, while you are doing the compass surveying, are such as an iron chain, metallic wrist band or ear rings(metallic) that one might be wearing.

Other things such as an electric pole or electric wires may also produce local attraction. The needle is attracted to these objects, so this will deviate from the true direction of the magnetic meridian.
If local attraction is available at a station then all the readings taken from that station will have the same amount of the error, and we have to correct the readings to get the true results.

There are methods to get the corrections to be applied on the erroneous readings in the traversing. The two methods which are used in general will be discussed here briefly.

(1) In first method we have to find out the stations where no local attraction exists. To find out this we have to look for a line where the difference between the fore bearing and the back bearing is exactly equal to 180 degrees. If we find such line then that means the two end stations of that line are free from any local attraction. After finding that line we apply the correction to the bearings of the other lines.

(2) In the second method we find the line where there is no local attraction. We know that even if the local attraction is present at every station the measured included angles will not be incorrect and we can calculate them correctly. With the help of the  readings from the stations which are free from local attraction and the correct included angles we can find out the bearings of all the lines.

If we do not find any line where the both stations are free from the local attraction, we have to take the line where the error is minimum and then apply the mean correction to both the stations and then take them as the correct readings. After that start as usual.

If you want further assistance with the topic, please leave a comment.

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