In the construction of original maps the principal points to be considered are the "scale,' the tprojection,° the °topographical rep resentation° and the method of ttluplication or reproduction° for publication.
Scale.—The scale will depend upon the character of the survey and the data obtained thereby; the extent of the area to be repre sented on the map, and the process by which the map is to be reproduced. The original map should always be drawn on a scale sufficiently large to allow the draughtsman to plat the field notes easily and accurately. The difference be tween a large scale and a small scale and their relative effect on maps may be explained as fol In all cases the original mapsbe subse quently reduced to any desired scale by hand, by pantagraph or by photographic methods. Re duction by pantagraph is satisfactory and useful only for the roughest classes of work. Photo graphic reductions of maps of limited size and prepared as copies for engravings may be made with fair accuracy and very slight distortion. Map sheets exceeding 15 to 20 inches square in the size of the final reduction cannot be reduced photographically with sufficient accuracy to satisfy the demands of absolutely accurate cop per, steel and stone engravings. For such en gravings the reductions should always be made by hand. The method of hand reduction usually employed is the simple but effective one of dividing the original and the copy into a system of equivalent squares and then drawing in all the details by hand. The very simplicity of the lows: Suppose the fractional scale of a map is given as it indicates that an interval of one inch on the map is equivalent to an interval or distance of 63,360 inches or one mile on the ground. Suppose the map to be 10 inches square, it would include an area of 100 square miles. Now suppose that while the size of the map remains the same the scale is reduced to ram, it would then include an area of 10,000 square miles. Any map scale which requires that the platting of measurements be made by the use of a scale with finer divisions than one hundredths of an inch is about the minimum limit for original work, although meas urements requiring an accuracy of one two hundredths of an inch are commonly made in the construction of accurate projections for the mapping of exact trigonometrical surveys. The
scale should be made larger directly in propor tion to the increase in the amount of details to be shown on the map. The scale for maps in railroad work should always be large-1 inch to 1, 2 or 300 feet. For the platting of plane table and other triangulations, and for work requiring the filling in of a great amount of topographical detail, the scale should not be less than one inch to one mile. All surveys such as the townships and sections of the Public Land Surveys, in which die measurements are made by chain or by should not be platted on a scale less than 1 inch to 40 chains or half a mile.
method demands the services of the most skil ful and experienced draughtsmen, so that the resulting maps possess a far greater accuracy than any reductions that may be obtained by photo-mechanical methods. There are examples of hand-made maps in the office of the United States Coast and Geodetic Survey which are actually superior in accuracy, execution and beauty to the prints from the very fine copper engravings for which they served as a copy.
If the maps are to be reproduced by being engraved on copper, steel or stone the reduc tions furnished the engravers should always be upon the efinal scale° or scale of publication. This is also applicable to wax engravings when photographic methods of reduction are not available to the engraver. When the maps rep resent a great diversity of physical character istics it as convenient to avoid confusion of details by using various conventional colors so as to distinguish the land from the water areas, the rivers and creeks from the roads, railroads, contour lines, etc. If the maps are to be repro duced by any of the photo-meehanical processes such as photo-engraving, photo-lithography or the half-tone process, the use of colors should be strictly avoided, and the map made in black and white on a scale that will allow about one-third reduction to the scale of publication.