This combination of colors renders these topographic maps readily legible. On the reverse of each sheet is a description of the mode of reading the map, and a legend, or series of conventional signs, indicating how the various facts shown on the map are represented. All these conventions are self-explanatory and are readily understood and interpreted by the layman, except, perhaps, the brown "contour" lines.

These contours are lines of equal elevation-lines along which the ground would be touched by the border of a water surface (of the ocean, for instance) if it were repeatedly raised by a given amount.

FIG. 1.-Ideal view and corresponding contour map.

Contour lines express three features of relief-(1) elevation, (2) horizontal form, and (3) grade or slope. To explain more clearly the manner in which the contours shown on the maps of the Geological Survey delineate height, form, and slope, the accompanying contour map (fig. 1) has been prepared from the ideal view shown above it. It may be interpreted as follows:

1. A contour indicates a certain height above sea level. In this illustration the contour interval is 50 feet; therefore the contours are drawn at 50, 100, 150, and 200 feet, and so on, above mean sea level. Along the 250-foot contour lie all points of the surface 250 feet above sea; along the 200-foot contour, all points that are 200 feet above sea,

and so on. In the space between any two contours are found elevations above the lower and below the higher contour. Thus the contour at 150 feet falls just below the edge of the terrace, while that at 200 feet lies above the terrace; therefore all points on the terrace are shown to be more than 150 but less than 200 feet above sea.

The summit of the higher hill is stated to be 670 feet above sea; accordingly the contour at 650 feet surrounds it. In this illustration all the contours are numbered, and those for 250 and 500 feet are accentuated by being made heavier. Usually it is not desirable to number all the contours, and then the accentuating and numbering of certain of them say, every fifth one-suffice, for the heights of others may be ascertained by counting up or down from a numbered contour.

. 2. Contours define the horizontal forms of slopes. Since contours are continuous horizontal lines, they wind smoothly about smooth surfaces, recede into all reentrant angles of ravines, and project in passing about prominences. These relations of contour curves and angles to forms of the landscape can be traced in the map and view.

3. Contours show the approximate grade of any slope. The altitudinal space between two contours is the same, whether they lie along a cliff or on a gentle slope; but to rise a given height on a gentle slope, one must go farther than on a steep slope, and therefore contours are far apart on gentle slopes and near together on steep ones. The topographic maps of the United States Geological Survey represent, besides the natural features mentioned, such artificial features as are of a public or more enduring nature, as State, county, township, and city boundary lines; reservation, land-grant, and publicland township lines; railways, streets, roads, and paths; bridges, ferries, dams, locks, and wharves; the location of permanent bench marks and triangulation stations, and the positions of light-ships and light-houses, mines, shafts, and tunnels. The only private features shown are houses, factories, stores, etc., no barns or outbuildings being represented.

The uses of such topographic maps are many. For the purposes of the National Government and the State they are invaluable, as they furnish data from which may be determined the value of projects for highway improvement, for railways, for city water supply and sewerage, and for the subdivision into counties, townships, etc. They serve the military departments of the Government, National and State, in locating encampment grounds, in planning practice or actual operations in the field, and, during war, in indicating the precise situations of ravines, ditches, buildings, etc. The Post-Office Department utilizes them in considering all problems connected with the changing of mail routes, star routes, and especially in connection with contracts and assignments of rural free-delivery routes. As the outlines of wooded areas are to be indicated on these maps, National and State

foresters will find them invaluable as a base for classifying the woodlands and recording the nature and quantity of the various trees and the relation of the wooded areas to highways of transportation, as railways, streams, etc. These maps are, of course, essential to detailed geologic studies and to investigations concerning mineral resources, water power, and land reclamation.

A good topographic map renders unnecessary a special survey for each new need. Prior to the existence of such maps every city was obliged to expend large sums in water-supply surveys. At far less cost the topographic map shows not only all these important local features, but also the relations between the artificial features in the immediate neighborhood and the topography and culture of the surrounding country, and thus broadens the scope of every such investigation.

In 1884 cooperation in public surveys between the Federal and State governments was proposed by the State of Massachusetts. In the year mentioned the legislature of that State passed a resolution providing for "a topographical survey and map of the Commonwealth" in cooperation with the United States Geological Survey, and appropriated the sum of $40,000 for carrying on the same. The Federal Survey allotted an equal sum to the work, and, in consultation with the State commissioners, agreed to make its surveys in the field on the larger scale of 2 inches to the mile, and to publish the results in separate atlas sheets having a scale of 1:62500, or about 1 mile to the inch, and with 20-foot contours. Under this arrangement the survey of the State of Massachusetts was completed in 1888. In the same year, 1884, a somewhat similar arrangement was made with the State of New Jersey, and the survey of that State was completed in 1887. In 1885 cooperation was arranged with the State of Rhode Island, and the survey of that State was completed in that year. In 1889 the cooperative survey of Connecticut was commenced, and it was completed in 1891. At the present time cooperative topographic work is progressing in New York, Pennsylvania, Ohio, West Virginia, Maryland, Maine, Kentucky, Alabama, Louisiana, Michigan, and California.

The general plan adopted for mapping the United States consists in dividing the country into quadrilateral areas (called "quadrangles") bounded by parallels and meridians of latitude and longitude. Three regular scales and a series of special scales have been adopted. For reconnaissance maps in the rougher portions of the Far West and Alaska the scale of 1:250000, or nearly 4 miles to the inch, is standard. For the general map of the United States the scale of 1:125000, or nearly 2 miles to the inch, is standard. In the more densely populated districts the Atlantic coast, parts of the Central region, and portions of the Pacific coast-the scale of 1:62500, or about 1 mile to the inch, is standard. These scales are readily convertible into the metric and other systems understood in foreign countries. However,

at the bottom of each sheet the scale is expressed in three ways-by a graduated line representing miles and parts of miles in English inches; by a similar line indicating distance in the metric system, and by a fraction. Special larger scales, from 2,000 feet to the inch up to 100 feet to the inch, are used for detailed surveys of important mining regions, for the planning and construction of irrigation projects, etc.

The maps are printed on sheets approximately 13 by 17 inches. The area represented on a sheet on the 1-mile scale is 15 minutes of latitude and of longitude, or, on the average, about 225 square miles; on the 2-mile scale, 30 minutes of latitude and of longitude, or about 1,000 square miles; and on the reconnaissance or 4-mile scale, 1 degree of latitude and of longitude, or approximately 4,000 square miles. An atlas sheet is known by the name of the most important town, city, or natural feature within the quadrangle represented.

The methods of field work followed in making these topographic surveys are as follows: At rather distant intervals throughout the territory under survey, say 10 to 25 miles, monuments are established, the positions of which are accurately determined by trigonometric methods and recorded in degrees of latitude and longitude. For this purpose base lines from 4 to 6 miles in length are measured with great accuracy, the location of one end of each base and of its azimuth being determined astronomically. From this base a network of triangulation is expanded to hilltops, mountain summits, and other prominent objects, the positions of which are carefully determined by computation. This is the special work of the section of triangulation and computing (see p. 64). All existing triangulation by other organizations is utilized. Where the nature of the country renders the extension of triangulation impracticable, primary control is extended by means of careful lines of traverse. The details of the methods employed are elaborated in another portion of this pamphlet (pp. 65–69).

The resultant positions are plotted accurately, by latitude and longitude, on large sheets of drawing paper, approximately 18 by 24 inches in size, called plane-table sheets. To each topographer is assigned, as his season's work, the mapping of one or more quadrangles, and he enters the field equipped with the plane-table sheet, which has on it only the trigonometric positions. A field party usually consists, in addition to the topographer in charge, of one skilled assistant, of the grade of assistant topographer or topographic aid, and several surveyors temporarily employed as traversemen or levelmen, with the necessary rodmen, teamsters, cooks, and other camp hands where subsistence is by camping.

The levelmen proceed to run lines of spirit levels over the various roads and paths, to determine elevations. The higher order of primary leveling, elsewhere described, is run at such distances apart that

at least two permanent bench marks may be established in each township or equivalent area 6 miles square. Levels of a less degree of accuracy are then run over every road, and where these are too far apart, say a mile or more, the levels are run over trails or across country, either by spirit level or by vertical angulation with stadia.

Meantime the topographer is extending a graphic triangulation on his plane-table board, the object of which is to locate precisely, on his sheet, points all over the area under survey. These include signals placed upon prominent hills, church spires, lone trees, and other definite objects; or in case the country is of such nature that it can not be controlled by triangulation, he has careful lines of traverse, by stadia or steel tape, run at intervals of about 6 miles and connected with the lines of primary traverse previously run across the sheet. At the same time the traversemen, equipped with lighter plane-table boards on which are smaller sheets representing about one-fourth of the whole quadrangle, are running graphic traverse lines over all roads and paths and across country, with a view to locating the cultural features, the courses of streams, the outlines of lakes, etc. On highways the distances are measured by counting the revolutions of a wheel, the circumference of which is known, or by stadia, and in dense woods by tape.

On the completion of this primary control and coincident with its preparation the topographer transfers to a clean sketch sheet the primary control points and adjusts to these the positions determined by secondary plane-table triangulation or stadia or tape traverse. The less accurate traverse lines are then adjusted between these more accurate positions and the elevations procured by spirit leveling or vertical triangulation are then added to the map.

Equipped with this control sheet, on which are shown approximately one trigonometric location per square inch of paper, two or three linear inches of traverse per square inch of paper, and from two to ten elevations per square inch of paper, the topographer walks or rides over all roads and paths and about the margins of lakes, across country, etc., selecting routes so near one another that he may be able to see all portions of the land. If the country is rough he uses an aneroid, setting it at one of the fixed elevations and frequently checking it by others of like kind; with it he is able to determine the crossing of each contour line on his route of travel. In a relatively flat country he uses a hand level, or reads vertical angles from his plane table and ascertains like facts. Stopping frequently, he sketches in, by means of contour lines, the relief of the country, and corrects as he goes the positions of roads, houses, streams, etc., located by his assistants.

For locations of topographic parties in 1903-4 see Pl. IV.

At the close of the field season the temporary employees are discharged and the party chief returns to the office. Here, during the