PRACTICAL NETIIODS. The construction of the necessary basic projection, as a iffcliluitiary to the making of a map is 110W a very simple mat ter, since the WOO: is accomplished by merely laying oil tabular values computed for the gen eral use of map-makers. Tables for these pro jections are easily obtained; those computed and published by the United States Coast and Geo detic Survey for various scales in meters and inches and by the Hydrographic Office of the United States Navy Department are the most useful for the purpose.
The preparation of an exact map presupposes a corresponding exact survey. For all maps of permanent value the survey must be based upon careful geodetic triangulations and levelings. For less exact work there are corresponding styles of maps, such as the plattings of sections and town ships by the United States Land Office; the general maps of the counties, compiled by county sur veyors by the use of the pedometer and the sur veyor's compass; the rapid military reconnais sance in which the engineer officer, note-book in hand, sketches in such features as may affect military the elab orate orate inaps of the United States Geological Survey, which undertake to give mi nute details as to geology, mines, forests, and topog raphy; and the perfect hydro graphie maps of the Coast and Geodetic Survey.
The maps of the Coast and Geodetic Survey are mainly hydrographie in their char acter. The original maps are projected on a polyeonie base, and on this base the elaborate system of triangulations con necting carefully measured base - lines is platted from the field notes by skillful draughtsmen. The main points of the coasts being thus indi cated, the interlying areas of the triangles are worked in from the notes of surveys made by stadia, chain, or tape; and the hydrographic data obtained by careful soundings along definite lines are also entered. The base map thus prepared is reduced by hand to the scale of publication. and a finished map is prepared as a guide for the engraver. On this map the hydrography is indicated by uniform signs. The shoals and sandbars are represented by dots, dose together along the shore and wider apart as they fall away into deeper water.. Lighthouses are indicated in their exact posi tions, together with their hearings from im portant points and their relation to channel entrances, etc. In short, all information re lating to the hydrography of the seaboard is carefully marked by appropriate signs consis tent on all the maps. From the finished map the engraver makes a tracing on hard gela tin sheets, which he transfers in reverse to a copper plate from Willa the ultimate prints are obtained. In late years. in order to
satisfy the increased demand for maps. a great deal of the hand reduction has been super seded by photographic methods, and lithographs have taken the place of the beautiful copper-plate print.
The general scope of the work of the United States Geological Survey is the surveying and mapping of the entire territory of the United States to obtain basic topographic maps for the exhibition of geological data. Each square de gree. called a 'reetangle,' in which the country is divided, is surveyed and mapped separately. (See Sunvfrisi:.) The Retailed information thus ob tained in the lield survey is roughly inked in at the close of the season. and then turned over to the photographer for reduction to the scale of publication, From the reduced photographic copies engravings arc made on stone, each sheet requiring three separate stone engravings. From the engraved stones transfers are unite to other stones and the sheets printed on a lithographic press.
The map shown on the accompanying plate has been designed to illustrate the methods of delineaticm employed by the United States ('oast and ueodetie Survey and United States Geologi cal Survey in the preparation of their charts and topographical maps. The map is. of course. ideal. and shows the use of the various conven tional signs. In the upper right-hand corner is a compass card indicating the true north and the magnetic variation of the particular locality, while the depths of the ocean are given in fath oms. The shoals are indicated as already ex plained, and also the lighthouses, rocks, in-aeons, buoys, etc•. The contour lines which form such a prominent feature of a topographical map connect all places at the same height above sea-level, and the interval between them is 50 feet. darker lines being drawn at the intervals of 250 feet. (See Co: mints.) in the topographical maps of the 'United States Government the contour interval is generally 20 feet. with heavier lines marking every 100 feet. In the maps of the Geological Survey these contour lines or relief figures are in brown distinguith them from the drainage, which is in blue. Such cultural features as buildings, roads, trails, railroads, tunnels. ferries, and bridges all have their appropriate markings, which are shown in the map. Fresh marshes arc distin guished from salt marshes by different enliven tional signs, while wooded country is shown in the lower right-hand corner of the map. Triangu lation stations are marked by a A and bench marks by an N. Alines and quarries, mine tun nels and shafts, also have their appropriate signs ac indivated.