Technical Definitions
Angle. Is the difference quoted in degrees between two bearings ; i. e., the angle of a true north bearing and a northeast bearing, is 45 degrees.Base or Base-Line. A carefully and accurately measured line upon which a triangulation depends.
Bearing. Is the direction of any object from the observer,
measured by the number of degrees from the true north (after corrections have been made for the variation of the compass). In every case the angle is always measured from north by east and south, or the same way the hands of a watch revolve.
Bench Mark. A stone placed to mark a level accurately fixed
by instruments.
Contour. A contour is the representation on a map of an
imaginary line running along the surface of the ground at the same height above mean sea level throughout its length.
Datum. Or Datum Level is an assumed level with reference to which heights are measured, or compared. In most cases sea level is used.
Dead Ground. Ground in which the observer could not see the
desired object.
Defile. A portion of the route where troops have to
reduce their frontage in order to pass, e. g., a mountain pass, bridge, an embankment, an archway or gate.
Fall. The drop in elevation of a river during its
course, usually measured in feet per mile; 2 feet per mile fall.
The form of the ground
A steep slope is one with a large angle between the sur face and the horizontal, and a gentle slope is one with a small angle. An even slope is one where the angle is constant, and an uneven slope is one where the angle changes. A level piece of ground is one which has the same elevation at all places. A flat piece of ground is not necessarily level, the surface of a board for instance is flat in any position. The terms valley, ridge, gulley, etc., are well enough known so description here may be omitted.There are two methods used on maps for showing the form of the ground. The most common is the Contour method, and the other, now very seldom used except on very small scale maps, is the Hachure method.
Contours should be thought of as lines cut Contours from the earth by a series of imaginary level
surfaces, with equal vertical distances be tween them. The French call them "courbes horizontales reprcsentant le terrain." The vertical distance between the imaginary surfaces is called the "Contour Interval" or some times the "Vertical Interval." (Abbreviated Contour C. I. or V. I.) Since a contour is a line in a Interval level surface, all points on it are of the same elevation or contours are lines joining points of the same elevation.
Map scales
THE DISTANCE shown on a map between two points is always the horizontal distance. The map is made as tho the observer were vertically above each point. This will be made clearer when the study of contours is reached.The scale of a map is the ratio between the Kinds of length of the lines on the map and the length Scales of the lines they represent on the ground.
There are three ways of showing the scale of a map : The Graphical Scale ; the Words and Figures Scale; and the Natural Scale or Representative Fraction, commonly spoken of as the "Scale" or the "R. F."
The Graphical Scale is the most common The Graphi- and best known, and usually appears in con cal Scale junction with one of the other methods. It is simply a line or graph marked off in some common units, such as miles, yards, meters, kilometers. In Figure 1, the distance from A to B is 200 yards. It means that a space A-B on the map represents 200 yards on the ground. Similarly CD is 540 yards.
When speaking of the scale of a map or cal The "Words culating distances on it the "Words and and Figures" Figures" method is usually used. "6 inches Scale = 1 mile" obviously means that 6" on the map represent a distance of one mile on the ground. "10 cm. = 1 km." (See Table II on page 11) means that a space of 10 cm. on the map represents a dis tance of one kilometer on the ground. It is awkward for a man trained in the English system to use a map in the Metric system, and vice versa, and for that reason most mili tary and topographic maps have their scales given in the "Natural Scale" or "Representative Fraction" method.