Where the primary object of the survey is to obtain a road-map, the road is gone over by the reconnoitrer and mapped directly on the paper. He indicates also. as a rule, the position of the most prominent objects within range of the road, noting the names of places. Collateral informa tion as to distances, water-supply, camping grounds, food, fuel, etc., may also be given. t nder other conditions, positions favorable for offense and defense are noted. The character of the roads so far as these may affect the move ment of troops is recorded. Information rela tive to intersecting watercourses. railroads, and bridges is usually added. After practice an officer of ordinary aptitude can survey and map in this way nearly as much road as he can traverse at the assumed gait. Thirty miles have frequently been mapped in a single day with sufficient accuracy for the desired purposes. These maps contain inaccuracies which, in an ex tended survey, may accumulate and become troublesome. Various means are resorted to for eo(O•linating arid combining the maps of adjacent territory which have been made by different offi cers. During the Civil War the general maps of the Peninsula were compiled almost entirely from reconnoissances on horseback. The rail road accurately surveyed and served as a basis, while the wagon-roads were mapped by different parties whose trails frequently crossed each other. In this way it was practicable so to adjust the errors as to obtain a general map of considerable accuracy. In General Sherman's march to the sea the topographical parties, were kept a day in advance of the army, were enabled to furnish maps for the guidance of the commanding general and his subordinates throughout the entire march.
In case something more than a road-map is desired. as for example a map of a possible battlefield, much more topographic detail should be shown on the map. So far as practicable, ever•tInng which might have any material bear ing in the case of a battle should he indicated.
For example, in addition to all roads suitable for the movement of troops, there should he shown the (1(.1)111 of cuts, position of stone walls, woeds, buildings, marshy ground, relative dis tan•s of level of hills and hollows, etc., etc. Any gusel engineer or snrveyor can learn in a few 'lays 90 to adapt his methods as to be able to prepare a fair military map. With additional practice his skill and judvment as to what should be added and What 0111111(41 in view of the limilalions. as 1(1 Si'1e of map and lime for making it, should improve greatly. The usual nut led of indicating di tames of level by means of contour., which are lines passing through points of equal height, is the best for the purpose and should be used wherever practicable.
In order that a map may be of the greatest service, it is desirable that a number of copies of it should be prepared and placed in the hands of the commanding general and his subordinates at the earliest moment after the completion of the survey. Several methods are in use for this purpose. One of the simplest is to trace the map tracing-linen; from this, if daylight be avail able, a number of prints can be made by the ordinary blue-print process. This work can be expedited by making from the tracing several brown-print negatives, which can in turn be used simultaneously if there be sufficient apparatus for making prints. if daylight be not available, prints may similarly be made by artificial light on bromide paper. Map duplication has also been effected with the aid of photography. A negative of the original map, usually on a smaller scale, is made with a camera. This nega tive is then used for printing positives b' any of the usual photographic processes. For further detailed information on this subject, consult articles in Report on Proceedings of Interna tional Congress of Engineers, Division of Mili tary Engineering (Chicago, 1S93) ; and Root, Military Topography and Sketching (Kansas City, 1896).