Graphical representation
The amount and character of the information given on a map depend upon the use to which the map is to be put and the size or the scale of the map. It is obvious that more detail can be shown on a large than on a small map of the same area. The features to be left off the map are deter mined by the use which is to be made of it. For instance, the ordinary County Map shows only property lines, streams and dwellings. The map found in a railroad time-table does not show types of bridges along the line, or whether the highways are metaled or not, but it does show the sequence of the towns and villages passed thru, the large streams crossed, and connections which can be made with other railroads. Maps of these two types rarely show differences in elevation. That information is not essential to their purpose. The topographic map, on the other hand, goes into detail as to the exact form of the ground, the location of buildings, fences, highways, rail roads, and other natural and artificial features.A military map is an elaborate topographic map. There is nothing on the surface of the earth which does not have its military significance, and therefore a good military map shows all the information which is compatable with its size. For instance the "Trench" or "Position" Map (See page 15) shows, first, the exact form of the ground hills, valleys, ridges, etc., whether the railroads are single or double track, steam or electric ; whether the highways are metaled, ordi nary county roads, or simply trails ; whether the fences are stone, hedge, barbed wire, smooth wire, rail, or board ; whether the bridges are truss, arch, suspension, ponton, foot or aqueducts, whether the buildings are dwellings, barns, factories, post offices, churches, telegraph offices, or military headquarters; where the telephone and telegraph lines are; the electric power transmision lines ; whether the streams are fordable ; where the woods are ; where the different mili tary units are located, etc., etc.
The term "Map" implies an accurately made drawing from a survey in which the distances and directions have been carefully measured with instruments. The name "Sketch" is given to the map which has been hastily made in the field by measuring the distances by some crude method such as counting paces, timing the trotting of a horse, or counting the revolutions of a wheel. Sketches are rarely used in modern warfare, except in minor operations. Topo graphic maps obtained in time of peace are converted into military maps by the use of aerial photography and data gotten by secret agents in the enemy's country. Extreme accuracy may be gotten from aerial phcko graphs "when the altitude of exposure is known, the focal length of the lens, and the size of the negative. Even when this data is miss ing, if two or more features can be identified from a good map, the relative distances of other features may be easily and accurately filled in.
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