Contour lines (see [page 119]) pass through points of equal elevation, and are spaced apart according to a predetermined plan, to indicate intervals in elevation of five, ten, or twenty feet, as may be desired. This predetermined contour interval has no necessary relation to the scale to which the map is drawn. Two otherwise identical maps of the same area may be provided with contour lines, one at the interval of five feet, the other at the interval of twenty-five.
A skilled map-maker, observing a slope, is able to sketch contour lines, freehand, with an accuracy sufficient for most purposes. But such skill is the result of much careful measured work.
In plotting contour lines it is best to work, not from line to line—errors of observation then accumulate—but to measure the altitude and the mean inclination of the whole mountain side, and go from the over-all measurements to the minutiae.
In drawing the contour of a mountain, rays may be laid by compass from the summit along ridges and through valleys, and then minute observations may be made along those several lines. The sweep of the contour lines between the points plotted along the rays may be filled in freehand, with the mountain side spread in view.
The data necessary for contour lines may be got by the use of the slope board alone; for, manifestly, at any certain angle, a contour interval of ten feet means a certain distance between successive contour lines. But in plotting contour lines, an aneroid is invaluable; with it one measures directly differences in elevation, and measuring thus the altitude of a slope, from bottom to top, the number of contour lines requisite may immediately be known; it remains to determine their distribution. Here observation, calculation, and experience combine to afford the result.
An aneroid should be used only under settled conditions of weather; and, even so, correction should be made, when possible, by taking the average of many readings of the same range.
It is not necessary to go afield with sketching board and its accessories. A map-maker who has taught himself a well-regulated stride may, when equipped with compass and notebook (and, if conditions require it, with an aneroid), collect all the necessary data; and then, subsequently, at home he may draw his map. It should here be said that, if one is going to gather data for map making after the manner just suggested, his compass should be one having a delicately mounted needle. It may advantageously be equipped with sights, and the scale should be reasonably large and the graduation minute. It should, in short, be a surveyor’s compass.
For more explicit instruction, the reader is referred to the manuals on Military Map Making. One by Major C. O. Sherrill, published by George Banta Publishing Company of Menasha, Wisconsin, is excellent. It should, however, be remembered that the ideal military map is one for particular needs, of maximum accuracy, based on a minimum amount of observation; timesaving is an important factor. Making proper allowance, the military manual affords all needed instruction and advice.