Projection.— At the present time the con struction of the basic projection for a map is a very simple matter and consists merely in the laying off of the proper tabular values which have been computed for the use of map draughtsmen. The most useful tables of this kind in this country are those published by the United States Coast and Geodetic Survey and the Hydrographic Office of the Navy Depart ment. The Mercator and the polyconic are the two principal projections employed at the present time, but many others are often em ployed where their special characteristics satisfy the particular requirements of the maps. It is in such cases that an intimate knowledge of the various projections is valuable to the map maker.
For information relative to the particular characteristics of the various projections see article under the title PROJECTION, and the value of an interval adopted — 10, 20, 50 or more feet — depends upon the purpose for which the survey is made, and the scale of the final map. In surveys for hydraulic purposes such as the construction of dams and reservoirs, the laying of water and sewer systems and the digging of canals, many cases occur which re quire the adoption of a contour interval as small as one or two feet. In such cases the areas surveyed are small and are mapped on a large scale. On the other hand, when the areas are large with great differences of elevation, larger contour intervals are adopted, and the maps are made on a small scale. The maps of the United Stales Geological Survey are among the best examples of orographic delineation by the method of contours, for detailed informa tion of which sec article under the title GEO LOGICAL SURVEY.
The delineation of orographical features on Topographical Representation.— The dif ferences of elevation between the various points on the surface of any region, that is, the hills, mountains and valley formations together with other natural features such as the river sys tems, plains, tablelands, etc., constitute the topography of that region. The term aorogra phyo is applied to the aspect furnished by those natural features the existence of which are due solely to differences of elevation —the valleys, hills and mountains. These are represented on maps either by a system of horizontal lines drawn at right angles to the slopes called %con or by a system of vertical lines called hachures Which follow the directions of the slopes and are drawn at right angles to the ((contours? The best idea of the nature of contours may be had by supposing a dam built across a river and flooding the country above it. Suppose the height of the dam to be in creased at uniform intervals of 10 feet. At each increase in the height of the dam the water will reach a new level, and the irregular horizontal line where it intersects the surface of the ground is the contour for that particular height of the dam and connects all the points of the same elevation on the surface of the whole region. In the actual determination of
contours the datum, zero or point of beginning of the vertical intervals is the level of the sea, the more accurate and useful classes of maps by the method of hachures is now being rapidly supplanted by that of contours. The only prac tical value of the former lies in its pictorial effect, and that, when viewed from the stand point of usefulness, fails to compensate for the enormous expenditure of labor and time in the execution of the maps and the subsequent en gravings. The most beautiful examples of this class of maps in the world are those that have been published from time to time by the United States Coast and Geodetic Survey, but even in the work of that bureau the method of hachures is being slowly supplanted, by the method of contours.
Duplication or Reproduction.— Maps are duplicated or reproduced for publication by be ing engraved by hand on copper, steel, stone, wood and wax. Wood and steel engravings are very rarely made at the present time, wax engraving being the most common process. Maps are also reproduced quite extensively by the photo-engraving and photo-lithographic processes. The finest classes of engraved maps, like those of the United States Coast and Geodotic Survey, are impression prints taken directly from the engraved plate by means of hand presses. In this bureau the engravings are made exclusively on copper plates. The maps of the United States Geological Survey, which represent the finest class of lithographed maps in this or any other country, are engraved on stone, from which impression prints are taken on hand presses and transferred to smooth stones from which the final prints are made on power-driven cylinder presses. A great many of the maps which make up the best classes of atlases published for scientific and other pur poses in foreign countries are reproduced in the same manner.
For detailed information relative to the various processes enumerated, see articles under the titles ENGRAVINGS, LrrsooltaPav, PHOTO ENGRAVING, etc.
Consult Craig, Treatise on (United States Coast and Geodetic Survey, Washington 1882) ; Gelcich and Sauter, tenkunde geschichtlieh dargestellt) (Stuttgart 1897); Gretsehel, (Lehrbuch der Kartenprojek tion> (Weimar 1873) ; Verner, (Map Reading and Elementary Field Sketching) (London 1893); Zondervan, (Leipzig 1901) ; West, Elements of Mili tary Topography) (London 1894) • Woodward, (Geographical Tables) (Washington 1894); Steinhauser, der Mathematischen Geographic and (3d ed., Vienna 1887) •, Fiorini, (Le Projeziom Belle carte Geografiche) (Bologna 1881) ; Cebrian and Los Arcos, (Teoria General de las Proyec ciones (Madrid 1895) ; Tissot, (Memoire sur la Representation des Surfaces et les Projections des Cartes (Paris 1881).