Colliery Surveying: A Primer Designed for the Use of Students and Colliery Manager Aspirants by Thomas Aloysius O 'Donahue

Author:Thomas Aloysius O 'Donahue
Language: eng
Format: epub
Publisher: Macmillan
Published: 1896-03-25T05:00:00+00:00

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Fig. 97.

down. The method, however, is not much used, as a fast needle survey can be made with only the same amount of trouble and with greater accuracy, but circumstances may arise, such as the loss of the clamping screw, which render a fast needle survey impossible, in which case the method, which may be termed the "correctingdeflection" method, may be advantageously adopted. Take Fig. 98 as an example of a roadway which is to be surveyed, and the point A is the only one at which the absence of iron renders a correct bearing possible. The instrument is

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fixed at A, and a bearing is taken towards B, which we will presume to be as far as a light can be conveniently seen from A. The bearing being read off, it is found to be N. 60 W., and as there is no attraction on the needle this is a correct bearing, and is booked accordingly. The remaining bearing of the roads

SURVEYING WITH THE MAGNETIC NEEDLE

83

must be calculated from this. The dial is now removed and fixed at B, directly under the mark at which the sight lamp was held, and a back bearing is taken to A, This should read N. 50 W., as it is the same line of sight as was previously taken, but owing to the local attraction of the needle the bearing is found to read N. 47 W. Thus it is plain that the needle has been attracted three degrees north of its true course. A fore-sight is next taken to C, and the bearing reads as N. 48 W. But as the position of the dial has not been altered the needle has been attracted three degrees north, therefore the correct bearing is 48 + 3 = N. 61 W. The instrument is now removed to C, and a back-sight is taken to B. This is found to read N. 53 W., and as the correct reading, as calculated from the correct bearing, is N. 51 W., the needle has been deflected two degrees to the west. A fore-sight is next taken to the end of the road D, and the bearing is read off as N. 56 W., but as the deflection is the same as was previously ascertained by the back-sight, namely, two degrees west, the correct bearing is N. 54 W. If the instrument were then placed at D, and the needle was free from disturbance, a back-sight to C should give the bearing N. 54 W., and a check on the work would be thus obtained.

It may so happen that a correct bearing cannot be taken at the commencement of the survey, in which case the readings of both back and fore-sights must b6 recorded and the correct bearings calculated when a bearing has been taken where the needle has not been attracted. The following shows a method of booking the foregoing survey, presuming D to be the only position at which a correct magnetic bearing can be obtained.

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