After direction and distance comes the interpretation of the signs, symbols, and abbreviations on the map. Those authorized are given in section 2 (a reprint of Appendix 4, Field Service Regulations, 1914); but there are a good many other conventional signs in common use. A key to them is published by the War Department, and is called "Conventional Signs, United States Army." From these you read at once the natural and artificial features of the country shown on your map. It should be borne in mind that these conventional signs are not necessarily drawn to scale, as are the distances. They show the position and outline of the features rather than the size. This, for the reason that many of the features shown, if drawn to scale, would be so small that one could not make them out except with a magnifying glass. If the exact dimensions are of any importance, they will be written in figures on the map. For instances, bridges.
In addition to te above conventional signs, we have contours to show the elevations, depressions, slope, and shape of the ground. Abroad, hachures are much used, but they serve only to indicate elevation, and, as compared to contours, are of little value. Contours resemble the lines shown in figure 4 (section 1)
Hachures are shown in figure 5 (section 1), and may be found on any European map. They simply show slopes, and, when carefully drawn, show steeper slopes by heavier shading and gentler slopes by the fainter hachures. The crest of the mountain is within the hachures. (See fig. 5, section 1.)
Contours.--A certain student, when asked by his instructor to define "space," said: "I have it, sir, in my head, but can not put it into words." The Instructor replied: "I suppose that under those circumstances, Mr. ----, the definition really would not help much." And so it is with contours--the definition does not help much if you know a contour when you meet it on a map. For examples of contours, turn to the map in section 2 and, starting at the United States penitentiary, note the smooth, flowing, irregular curved lines marked 880, 860, 840, 840, 860, etc.
The only other lines on the map that at all resemble contours are stream lines, like "Corral Creek," but the stream lines are readily distinguished from contours by the fact that they cross the contours squarely, while the contours run approximately parallel to each other. Note the stream line just to the west of South Merritt Hill.
The contours represent lines on the ground that are horizontal and whose meanderings follow the surface, just as the edge of a flood would follow the irregularities of the hills about it. Those lines that contours stand for are just as level as the water's edge of a lake, but horizontally they wander back and forth to just as great a degree.
The line marked 880, at the penitentiary, passes through on that particular piece of ground every point that is 880 feet above sea level. Should the Missouri River rise in flood to 880 feet, the penitentiary would be on an island, the edge of which is marked by the 880 contour.
Contours show several things; among them the height of the ground they cross. Usually the contour has labeled on it in figures the height above some starting point, called the datum plane--generally sea level. If, with a surveying instrument, you put in on a piece of ground a lot of stakes, each one of which is exactly the same height above sea level--that is, run a line of levels--then make a map showing the locution of the stakes, a line drawn on the map through all the stake positions is a contour and shows the position of all points of that particular height.
On any given map all contours are equally spaced in a vertical direction, and the map shows the location of a great number of points at certain fixed levels. If you know the vertical interval between any two adjacent contours, you know the vertical interval for all the contours on that map, for these intervals on a given map are all the same.
With reference to a point through which no contour passes, we can only say that the point in question is not higher than the next contour up the hill, nor lower than the next one down the hill. For the purposes of any problem, it is usual to assume that the ground slopes evenly between the two adjacent contours and that the vertical height of the point above the lower contour is proportional to its horizontal distance from the contour, as compared to the whole distance between the two contours. For instance, on the map, find the height of point A. The horizontal measurements are as shown on the map. The vertical distance between the contours is 20 feet. A is about one-quarter of the distance between the 800 and the 820 contours, and we assume its height to be one-quarter of 20 feet (5 feet) higher than 800 feet. So the height of A is 805 feet.