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Wellington Independent, Volume XXIV, Issue 2812, 20 March 1869, Page 2

Instructions for the Guidance of the Staff of Surveyors under the Provincial Government of Wellington. [Continued from March 6th page 2} [Concluded from Issue No. 2806.] direction of the water courses, ridges, and spurs, the watersheds, bush and other natural features, after a careful study with the aid of a pocket compass, may become faithfully depicted on the map. It should be borne in mind that rude sketches from memory consisting of caterpillar like daubs for mountain ranges and spurs and vermicular lines for streams unlike nature and untruthful in existence only disfigure the map and render it incorrect, whereas drawings from nature with the relative distances of the features accurately delineated enhance to a high degree both the beauty and usefulness of the plan. SURVEYING BY TRAVERSE. The system of surveying by traverse explained 48. After a sufficient number of Trigonometrical points have been fixed over a block of land undergoing survey, the delineation of its boundaries, streams, proposed road lines, and sectional subdivisions is to be operated upon by traversing, which is a system of measuring by a series of successive straight lines of various lengths and on dissimilar bearings, a circuitous route from any one point to another, however distant or placed with reference to each other. The sums of the meridian and perpendicular distances between the stations composing the traverse furnish data for the computation of the bearing and distance between the two points, and if these points are also trigonometrical stations of the survey it is obvious, if the work has been accurately performed, that the deduced bearing and distance from traverse should coincide with the trigonometrical values thereof. Again if when the traverse starts from a certain point, and after proceeding in a circuit it returns to this same point, then the sum of the distances gone north should be equal to that gone south, and the sums of the distances gone east and west should also be equal. Hence the traverse system furnishes an easy check for ascertaining the accuracy of the work, and thus practical limits to the errors committed may become assigned. Degree of accuracy attainable by the traverse system of surveying. 49. Upon open and tolerably level country, where it is possible to obtain fair length of lines between the traverse stations, and where no greot impediment to the horizontal lay of the chain is met with this system is susceptible of remarkable accuracy. The small unavoidable errors exhibited on computing the meridian and perpendicular distances ought not to exceed the limit of one link for every ten chains so traversed, and in the actual prosecution of such surveys they have proved not to exceed the one half of this amount. But over hilly and rough localities where the lines are necessarily short, and the horizontal measurement of a chain's length over the ground may be said at best to be equivocal, this process entirely fails, and should therefore only be employed for obtaining the points of intersection of section corners, and the lengths of those important lines when there is evidence of the practicability of applying the system, or in the absence of all other available methods. Rules for detecting errors in the bearings of the traverse lines, and for their elimination. 50. Euclid, Book I, Prop, XXXII, demonstrates, that any rectilineal figure can be divided into as many triangles as the figure contains sides, and consequently that the sum of the interior angles, plus 300° is equal to twice as many right angles as the figure has sides. The application of this theorem furnishes a complete check upon the angular measurements of the traverse. Thus if the theodolite is set up at a Trigonometrical station, with the axis of the telescope pointing to another one, whilst the vernier reads the known bearing between these two points, the bearing of the first or any station visible of the traverse is obtained in terms of the Trigonometrical Meridian. Then when the instrument is removed to the station so observed and set back on the Trigonometrical Station with the vernier still on the same reading the bearings of the adjacent stations are obtained, and thus successively the bearings between all the stations. The difference of bearing between any two adjacent stations is an angle of the rectilineal figure formed by the traverse. If then the so deduced bearing from the last station of the traverse to a second Trigonometrical Station arrived at differs somewhat from the correct bearing as obtained by resetting the instrument when on this second Trigonometrical Station to the former one left or to any in view, this difference will exhibit the amount of error committed in the measurements of the interior angles of the rectilineal figure before alluded to. The best method of expunging these errors is by equal dispersion throughout the traversed stations, and their limit should be confined to one minute for every two bearings. Manner of preserving an uniform meridian. 51. Care must therefore be taken to observe as many of the traverse stations as can be seen from the trigonometrical points so that by constantly referring back to such data for correction the dispersion of errors may be limited to their proper precincts ; and also in depressed localities such as ravines and beds of rivers and streams, where stations are liable to become numerous, long bearings should be thrown from point to point near to the traverse for constant and convenient reference, to preserve the uniform direction of the meridian. Detection of errors in the Chain Measurements of a Traverse. 52. If the above rules for preserving an uniform meridian throughout the traverse have been strictly attended to and provided distances have been fairly chained, the sum of northing and southing, or of easting and westing by traverse will be found to coincide nearly with the trigonometrical values in the case of a traverse performed between two points fixed by triangulation or to closely balance in instances when the traverses check by closing circuits. If the