THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CXVI. FRIDAY, JULY 19, 1918 No. 3003. The Future Aspect of Mine Surveying. By JOHN PROCTOR. The magnetic declination has already been charac- terised * by the author as very fickle, and certainly, from the surveyor’s standpoint, the term is fully w irranted, as it has let him down on many occasions, and been the cause of much bad surveying. We find tbat the magnetic declination varies annually, seasonally, diurn ally, and, during periods of storm, very erratically. Of the diurnal variation most surveyors had an idea; but since the weekly reports from the Meteorological Office have been published in the Colliery Guardian, the magnitude of this variation has been brought home to them more fully. Since, unfortunately, the period of greatest variation approximately coincides with the time most surveyors do their work, it will be evident that a surveyor who makes a survey in the early part of the week and wishes to plot it, is placed at a disadvantage. He knows there is an error which he should allow for, but must wait till he gets the weekly report on the Saturday before he can make the necessary allowance. Influence of Variation on Surveys. The variation itself raises new problems for the surveyor. When he starts a fast-needle survey—say, in the morning—he commences with a clear bearing free of attraction, then proceeds with his survey, and, in the course of a few hours or at the end of his survey, swings his needle and checks his vernier. Usually there is a difference, and we know now there should be a differ- ence. In the event of the difference being considerable, several problems arise—was the first clear bearing correct and the second one wrong, or was the second one correct and the first one wrong? was the error instrumental or due to bad surveying, or was it due to diurnal variation? These are points that have to be considered when using the magnetic compass. Compass Variation. Now, apart from terrestrial magnetism, we find that compasses themselves vary. Often there is as much as 20 min. difference between one compass and another, and, taking the same compass, the author has seen as much as 25 min difference between the north and south ends of the needle. This could be due to the needle not being perfectly straight, or not properly balanced, or to inaccurate graduation. Most surveyors are aware of this error, and stick to the north end, but it is question- able if this is always the correct thing to do. Influence of Extraneous Objects and Electricity. Apart from instrumental errors and errors due to magnetic declination, we find that the needle is influ- enced by rails, trams, conveyors, etc., by bands of iron- stone and balls of mine in the strata—and even bricks will attract the needle. The author tried the effect of placing half of an ordinary, brick as close as possible to the end of the needle, and the deflection was almost a degree. However, surveyors need not be alarmed on this point, as the magnetic force varies inversely as the square of the distance. Electricity, again, plays havoc with the needle, and the advent of large electrical plant at collieiies makes it necessary for surveyors to give this matter considera- tion. At Britannia Colliery the author has to pass within 20 yards of the power station using several thousand kw., and the needle is bound to be and is actually affected thereby. Underground there is a considerable amount of ironstone in the overlying .strata, and sometimes the attraction due to this will amount to several degrees—in fact, the needle “ sticks,” and can be made to read nearly anything. Then the colliery is worked by the latest methods, every yard of open face in the pit containing a conveyor, and as there are no horses in the pit, every road is a haulage road. Where there is a side road, it either contains a conveyor or pipe connections to the nearest face conveyor. Thus it is manifestly extremely difficult to set up the compass free of attraction. Setting off a Base Line. When the author commenced laying the colliery out off a base line, which will be described later, he did not foresee the difficulties peculiar to the system of mining adopted, but employed this method solely because he had made up his mind long ago as to the value of the magnetic needle. The author believes he has made out a strong case against the use of the magnetic needle, which has been responsible for much bad surveying—though not all of it. Now what is the alternative ? Bearing in mind that the object of mine surveying is to have the workings in correct relationship to them- selves and to the surface, the method that will give the most direct connection between the two is surely the best. 'this can be accomplished in two ways: By hanging two plumb lines in one shaft, or by hanging one plumb line in each of two shafts—the further apart the better. The former method has the disadvantage of a very short base line, and consequently calls for very great * Colliery Guardian, June 11, 1918, p. 1155. accuracy both in connecting up to the surface and transferring the base underground. In many cases, owing to obstructions in the shaft, the length of base that can be obtained is ridiculously small; whilst, in very deep shafts, it is difficult to get the plumb lines to steady, owing to the ventilation, and even if the fan be stopped—which is out of the question, for any length of time in fiery mines—there is still the air movement, due to motive column, which in deep mines may be consider- able. Should there be pumping stations in the shaft, the ventilation circulating through them, which has to turn through an angle of 90 degs., may have the effect of permanently displacing the plumb lines from the vertical. With a base line of 15 ft., and 1-in. error in plumbing, there would be an angular error of five minutes to commence with, so it will be obseived there is not much room for a factor of safety in plumbing. Moreover, the method has the disadvantage of not affording a check unless it be repeated in another shaft or connected up to a plumb line in that shaft. Sighting down the shaft is a modification of the above method which has the advantage of being independent of the ventilation and plumbing errors; but, on the other hand, the length of base line may be even more limited than with two plumb lines. This method calls for a special theodolite of very accurate construction, which is not within the reach of many surveyors. Method Adopted at Britannia Colliery. In the author’s opinion, the best method is to make use of the two shafts; and this is the plan adopted by the author at Britannia. The shafts are exactly 70 yds. apart, as checked on the surface. A connection having been driven between the two pits for ventilation, a theodolite survey was made between the pits, the centre of the winding rope being sighted. (At the time there was only one rope over the centre of each pit, the cages not having been yet put in). The distance and bearing of the centre line was then calculated in relation to the survey. This bearing was taken as zero, and the bearings of the survey wete altered accordingly, two marks being left to enable the author to continue the laying out of the pit. An allowance had previously been made for an error of 6 in. in plumbing, and this it was calculated would entail an error of 8 min., which could very well be afforded for a year or so, as the swing due to this error would not be much and would be relative to the surface. The underground workings would be laid out from a fixed base line, and consequently this error would not affect the drivages. In calculating the closing distance there was an error of 01 ft., that is to say, about 1J min., so far as the line of the pits was concerned; but, of course, there might have been an error, due to plumbing at right angles to the line of the pits, which the calculations would not disclose. However, the author was able to prove that he was not far out in this direction either. After a year or so it was decided to put rigid guides in one of the pits, starting from the bottom and working upwards from a depth of 720 yds. The correct centre line to commence with became of very great importance, having an important bearing on the efficiency of the winding engines, the wear and tear on the guides and shoes, and the general smoothness of the wind. It was decided to give the centre line with the compass in preference to plumbing, and in this matter allowance was made for an error of 2 in., which, taking the radius of the pit as 10 ft. 6 in., gave an angular error of 1 deg. roughly, or a factor of safety of 7 on the 8 min. error allowed on the underground work. Finding True Centre of Shaft. Of course the true centre of the pit had to be found, which was done by lowering the winding barrel into the sump and allowing it to steady; an operation which did not take long, as the winding rope was of the locked- coil type. The correct centre was then fixed off the winding rope by stringing lines, across the pit, to nails attached to boardings placed over the shaft. The centre obtained was checked roughly with measurements to the sides of the shaft. While the rope was being centred, the author surveyed round from a road near to the shaft, whose bearing in relation to the centre line of the pits was known; and by plumbing ti e compass over the centre of the shafts, and turning the sights on to 90 degs., the required centre line was obtained. After having g