A NEW DEVICE FOR TACHEOMETRIC PLANE-TABLE SURVEYING.
Hutin Louis · Minutes of the Proceedings of the Institution of Civil Engineers · 1917
SURVEYS are frequently required in new countries, where the rapid production of a plan is of more importance than a high degree of accuracy, as for instance for determining the best lines of communication by roads, railways, ropeways, etc., for laying out water-races, or for investigating a mining property, when it is necessary to map as promptly as possible a number of mineral outcrops, in order to determine where trial workings should be commenced.For such purposes errors of the order of even one per cent.are generally negligible, but speed is all-important.The value of the plane-table for such work is generally recognised, so also is that of the tacheometer, especially in remote countries, where it is impossible to obtain the services of reliable chain-men.In level country a plane-table and alidade fitted with a small telescope having stadia lines in the reticule can be used without any difficulty, but most mining work has to be done in hilly or broken country, where the inclined distances read off by the tacheometer require to be reduced to the horizontal.This reduction must be made in the field, and it has been performed hitherto in a number of different ways.Amongst automatic methods, the best known, at any rate abroad, is Ljungstrom's instrument, made in Stockholm, in which the elevation or depression of the telescope reduces automatically the foreshortening of the scalar distance, on the alidade, to the equivalent horizontal.The objections to this instrument are that the numerous moving portions are apt to wear in use, and that it is cumbersome, heavy, and expensive.Another device, due to Professor Jeffcott, and made by Messrs. Thomas Cooke and Sons, Limited, consists in causing the inclination of the telescope to alter automatically the distance between the stadia lines or points, so that the reduced horizontal distance