Projection of Routes.
The following directions for the projections of routes by Captain George, R.N., are so thoroughly plain and practical that it will be well for the travelling observer to have recourse to them:—For out-door or field work the easiest method is by the plane projection, the data thus obtained being transferred to a Mercator’s projection at the first halt or stopping station. In the plane projection one equal length is assigned to all the degrees of latitude and longitude. It was first adopted on the erroneous supposition that the earth’s surface is a plane; it is still the best for the traveller to use in his early attempts to project his journey while the objects are still in sight. This projection is available as far as 20° on either side of the Equator; beyond the parallel of 20° and as far as 60° Mercator’s projection is preferable. Between 60° and the pole the distortion of both the plane and Mercator’s projection is so apparent, that a polar or circular projection must be adopted. Sheets of paper, ruled into squares by strong lines, and subdivided by finer ones, afford great assistance in map work. For out-door work the scale of 1in. to one mile is amply large enough to register every particular of one day’s journey on a sheet of 12in. square. The in-door, or table plan, may be reduced ten miles to the inch, and plans for transmission home maybe again reduced to 1in. to 1° when larger plans cannot be sent. The chief point aimed at in the following directions is to draw more attention than has hitherto been given to the true bearing of objects, for the following reasons: First. Any object whose true bearing is east or west must be in the same latitude as the place of the observer. Secondly. Any object whose true bearing is north or south must be in the same longitude as the place of the observer.
While travelling in a northerly or southerly direction, from a station whose latitude is known, and carefully noting the distance and direction travelled, it is only necessary to watch when objects come to the true east or west, and their latitude is obtained. When travelling in an easterly or westerly direction from a fixed station, noting distance and direction, it is only necessary to watch when objects come to the true north or south, and their difference of longitude can be obtained by using Table B, from the station left. Thus, suppose a traveller passes from A, whose latitude is known, towards some distant hill (B), his route making an angle of 25° with the meridian. He sets his sextant to 65° (65° + 25° = 90°), or to 115° (180° - 65°); then as the objects 1, 2, 3, and 4 successively come into contact with B or A, as the case may be, he ascertains with precision the moment when they are truly east or west of him, and so, knowing the distance he has travelled from A, he can readily calculate or project their latitude.
When the traveller, as will frequently be the case, has to deviate from the line of route, his position can be determined by compass, or true bearing of any object, and an angle of a second object; or he may have recourse to transit observations; that is to say, wherever two fixed objects come in line, an angle to a third object will determine the position with great accuracy.
Observe that in travelling along X Y Z the hills A B C can be mapped for at X, or thereabouts; the bearing of B from C can be determined at Y; that of A from B; and at Z that of A from C, and so on, for any number of hills. And it is very important to recollect that it is not necessary to catch these lines of sight precisely, for by taking bearings twice, and the intermediate course approximately, there are sufficient data for protracting out upon paper the required bearing.
Thus, as soon as the peak of a distant hill is about to be occulted by the shoulder of a nearer one, a bearing should be taken, and again another as soon as it has reappeared on the other side, and the intermediate course noted.
The advantage of this method of filling up a field sketch will become more apparent as experience is gained. A third and accurate method of fixing the position is in general use among marine surveyors, but has hitherto been but little resorted to by land travellers, viz., by the angles subtended between three known objects. The instrument called the station-pointer is generally used for this purpose, but the position may also be found with a pair of compasses and a protractor, or more simply as follows by means of a protractor and a sheet of tracing paper. Draw a line through the centre of the paper, place the protractor on it, near to the bottom of the sheet, lay off the right hand angle to the right, and the left hand angle to the left of the centre line; rule pencil lines, radiating from the point over which the centre of the protractor had been placed, to the points that had been laid off, then place the paper on the plan or map, and move it about until the three lines coincide with the objects taken, prick through the points that lay beneath the centre of the protractor, and the observer’s position is transferred to the plan. When possible the centre object should be the nearest.