After reading this article you will learn about eight main precautions that should be taken while using a compass. Also learn about the sources of error in compass work.
1. The glass cover of the compass box gets charged with electricity when dusted off with a handkerchief or by the influence of electric charge in the atmosphere. Consequently, the needle adheres to the glass. This may be avoided by applying a moist finger or cloth to the glass.
2. To avoid local attraction, nothing made of iron or steel such as bunch of keys, iron-buttons etc. should be carried by the surveyor. The instrument should not, as far as possible, be set up near telegraph poles, rails, other steel structures and currents carrying wires etc.
3. Set up and level the compass properly.
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4. Stop the vibrations of the needle by gently pressing the brake-pin so that it may come to rest soon.
5. Always look along the needle and not across it, thus avoiding parallax. In Surveyor’s compass always read the north end of the needle.
6. For important lines, take duplicate readings at each station. After having taken the first reading, displace the needle and take the second reading and then take the mean of the two.
7. To detect local attraction, take fore and back bearings of the lines.
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8. The pivot sharp edge should be protected by keeping the needle off the pivot when compass is not in use or when it is shifted from one station to another.
The compass should be tested and adjusted as explained below before putting it to use:
1. When the compass is levelled the needle or compass ring should be horizontal, if not, slide the rider on the higher end of the needle to make it horizontal.
2. The needle in the compass should be straight and the pivot should be at the centre of the graduated circle. To test this, read two or three sets of the end readings of the needle.
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The difference for each set shall be exactly 180° for the needle being straight and pivot in the centres, otherwise, there may be any of the following two cases:
(i) The difference being constant for all sets.
(ii) The difference not being constant for all sets.
In the first case, the pivot is in the centre, only bend the needle straight. But in the second case, the correction is made by bending the pivot as well as by straightening the needle.
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3. The needle should be sensitive so that it may not come to rest in a direction other than the magnetic meridian. To ascertain if the needle is sluggish, take reading in any position of the needle being in rest. Then displace the needle by bringing near it a piece of steel or bunch of keys etc. and let it come to rest and then again take the reading.
The reading will be the same if there is no friction on the pivot and the needle is not sluggish. If reading is not the same, then the pivot-point should be sharpened by a fine oilstone and the needle should be remagnetised by a bar magnet.
4. To find if the sights are vertical when the compass is levelled, suspend a plumb-line in front of the compass and sight it. If the sights are vertical, the eye-vane, the object-vane, and the string will be parallel and in the same vertical line. If not, then either file one side of the bottom of the vane where it rests on the plate or insert a paper packing. Repeat the test and adjustment until the error is eliminated.
5. To see if the sights are fixed diametrically opposite to each other, stretch a fine horse-hair between the sights. It will pass over the N and S marks (zeros if the sights are fixed exactly opposite to each other).
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6. To detect if there be any error due to careless working or external influence, take the fore and back bearings of a line. These will differ exactly by 180° if the work is correct and there is no external influence.
Sources of Error in Compass Work:
The errors may be due to faulty instruments or bad observations or natural and other causes and accordingly they are classified as follows:
1. Instrumental Errors:
(i) The needle not being perfectly straight.
(ii) The needle being sluggish either by having lost its magnetism or due to dip, or friction on the pivot-point.
(iii) The pivot not being in the centre of the graduated ring.
(iv) The graduated ring not being horizontal.
(v) The sight vanes not being truly vertical.
(vi) The line of sight not passing through the centre of the graduated circle.
(vii) The horse hair being too thick or loose.
2. Observational or Personal Errors:
(i) Inaccurate centering of the compass over the station-point.
(ii) Inaccurate levelling of the compass.
{iii) Imperfect bisection of the object sighted.
(iv) Carelessness in reading and booking of the bearings.
3. Errors Due to External Influences:
(i) Local attraction due to presence of magnetic substances nearby the compass.
(ii) Magnetic changes in the atmosphere.
(iii) Regular and irregular magnetic variations.