TOPOGRAPHIC MAPS A topographic map is one which shows the general lay of the land surface. On it will appear all creeks and rivers, hills, valleys and mountains; houses and towns, roads and railroads; so that anyone capable of understanding and interpreting the maps will see a clear picture of the land so mapped.
The United States Geological Survey has undertaken the great task of making topographic maps of the entire country and at the present time about one-third of it has been so mapped. These maps are in general use and are considered as a standard in topographic mapping. (Fig. 32.) The country has been divided into rectangles known as Quadrangles, the boundaries of which are certain parallels and meridians, or as generally referred to as latitudes and longitudes. The location of the territory which the map shows is indicated by a name which has been given to it from some important town or feature that the territory may possess, or it may be known by the controlling meridians. The maps are made to three different scales, the largest one being a scale of ,r,thT„ by which is meant that a distance of one inch on the map will equal 62,3oo inches on the ground, or one inch on the map will nearly equal one mile on the ground. This scale is employed in sections of large population or important industrial centers. For the greater part of the country a scale of or one inch to nearly two miles, is used. In the desert regions of the Far West a scale of or four miles to an inch scale is used. For some important mining regions, special or larger scales have been employed to some extent.
The general scheme of representing the various surface features is based on the use of three colors, so that the works of man, known as culture, such as houses, railroads, roads, bridges and towns are drawn in black. Creeks, rivers, lakes, seas or water of any kind are shown by blue coloring, while the relief or the surface elevations and depressions, such as hills, mountains and valleys arc depicted by brown figures and brown contour lines. (In some of the latest maps, a fourth color, green, has been used to represent wooded areas.)
Various conventions arc used to represent certain features, and they may best be understood by a study of the accompany ing table of conventions, on which a number of the common symbols are shown. (Fig. 33.) Contour Lines. When points having the same elevations are connected by a line, the resulting line is known as a contour line. (Fig. 34.) If one follows a contour on the ground, he will be going neither up hill nor down hill but will maintain the same altitude all the time. So each line represents a certain elevation, which in the case of government topographic Maps are based on mean-tide as a datum plane. A scale of 20 feet contour interval indicates, that the difference of elevation from one line to the next one is 20 feet. By the use of these lines it is a simple process to show hills and valleys very plainly, not only their shape and form are shown but the differences in altitude are recorded by them. Certain lines, generally the fifth ones, show by the figures on them the elevation they represent; thus, the 50o foot contour will indicate the points on the earth's surface which are Soo feet above sea-level. The grade or steepness is also indicated by the contours, and upon observation it will be seen that when the lines are dose together they indicate steepness and when they are further apart the slope is a gentle one. It must be Understood that contour lines can never cross each other, and that they will always close upon themselves; this may take place in a small area or may be noticeable only if a large territory is examined.
Besides the contour lines, figures are also used to denote the elevations, thus prominent points, such as railroad or road crossings or prominent hills may be marked by brown figures indicating the elevation of that point to the nearest foot. Many bench marks (B.M.) or points of exact elevations have been established, and are marked by permanent metal tablets. Such elevations have been very carefully determined and known to the nearest thousandth of a foot.