SURVEYS, TOPOGRAPHICAL RECON NOISSANCE. AND 'MAPS. As indicated above, where a regular survey of a tract of land is desired for some military purpose, such as the construction of a fortification, or a map of a military reservation, the survey will be made in the same manlier and under the same limitations as to time expended, accuracy required, and in struments used, as a similar survey for any civil purpose. (See SURVEYING.) Where, however, a survey is desired for use in connection with the movements of troops. or with the planning of a battle, it is almost invariably found to be neces sary to resort to hastier methods. These must depend upon the nature of the information de sired and the time allowed. Much assistance can frequently he derived from existing maps of the territory to be traversed. These generally fall short of giving all the information desired, and when they are available must usually be sup !demented by topographical reconnoissances. The latter can be made either on foot or on horse. back. In general, the information to be ob tained is plotted as it is noted, with a view to having the map as nearly finished as may be when the route to be mapped has been traversed. The essentials for such a survey arc paper for the limp, an instrument for measuring distances, and an instrument for determining elevations and slopes. For many years such maps were, in the first place, plotted directly in note-books or on pieces of paper ruled in one direction and attached to a light drawing-board of such sire that it could he readily carried by the recon noitrer. The bearings of roads and (lbjects were taken by a prismatic compass, or by a pocket compass, and were plotted on the map with the aid of a small protractor. Distances are usually measured by counting the number of steps. Where a wheel of any kind accompanies the reconnoitrer, distances can be determined with reasonable accuracy by noting the number of rotations of the wheel by an odometer or other means. Differences in elevation of points are determined either by a hand level or by compu tation from the slopes which are measured with a hand clinometer. Many little devices assist
greatly in the rapid completion and easy under standing of such a map. For example, if at the end of the (lay's work a red pencil be run over the roads, a blue one over the rivers and other water surfaces, and a green over the forests, much will have been done to add to the clearness of the map.
It was by these means that many of the to pographical reconnoissances of the Civil War were made. The principal inconveniences of the method related to the size of the drawing-board and to the fact that provision must be made for carrying the various auxiliary instruments in convenient places about the person. In recent years some attempt has been made on the part of inventors to develop a sketching-case which shall combine all the essential tools required for a topographical reconnoissance. One of the best so far developed is shown in the accompany ing cut. It has been made possible in this ease to use a much smaller drawing-board because the paper can be quickly rolled back and forth over it by means of the rollers on two opposite sides. The protractor can likewise be moved from side to side in a transverse direction. These two motions permit any point of the paper to be brought under the centre of the protractor. The protractor can be revolved freely around its centre, thus permitting a line to he drawn in any direction through any point on the paper. The compass in one side of the board permits the board to be oriented, that is, placed in a position parallel to the one which it occupied in starting the survey. Holes are provided at one end of the board for holding pencils. A rough eh nometer has been added at the other end. The scale of maps made in the field is usually selected between one inch to the mile and six inches to the mile. Two inches to the mile has been found convenient for reconnoissances made on horse back, and four inches to the mile for foot recon noissances. The scale used should be clearly shown on the map. A north-and-south line, the name of the reconnoitrer, and a clear title should also be plainly indicated on it.