Going Afoot: A book on walking.

by Bayard Henderson Christy

Proceeding thus, the outstanding features of the area may be mapped, one after another. The intervening details may be filled in, freehand.

It will have been remarked that only very simple apparatus is required for map making: the sketching board may conveniently be mounted on a tripod, with provision for turning it evenly and surely. Boards so mounted and intended for the very purpose may be had of dealers in draftsmen’s and surveyors’ supplies. A level should be provided, for use in setting the board up. The ruler will be graduated to inches and fractions of inches, if the map is to be drawn to predetermined scale. In pacing, one must carefully count his strides. A pedometer may be used, but a pedometer is a sort of toy; it requires to be carefully adjusted to the stride of the user, and is hardly worth while for any purpose. It may be convenient in pacing to use a tally register, and so relieve one’s self of the necessity of keeping count.

The value of a map is vitally dependent on the accuracy with which it is made. Measurement and observation should be repeated, and errors eliminated by averaging variant readings.

Nothing has yet been said about a compass, and a compass, though not necessary, is so serviceable as to be almost indispensable. With a compass one can not only do, and do more expeditiously, what has thus far been described; he can do some things which could not otherwise be done.

A sketching board is ordinarily provided with a compass, set near its upper margin, and bears also an orientation line passing through the compass. The board is set up and leveled and then turned until the orientation line coincides with the line on which the needle points. At each station the board is oriented, not by sighting along penciled rays, but always in the manner described, by bringing it to a truly north and south position. In other respects, the plotting is performed in the manner already described.

Orientation by compass is advantageous in this respect: given two points, as a and b, on the map, the map-maker may plot a third point, as D for example, while standing at D, and without being obliged to go either to A or to B. He sets up his board at D, levels it, and orients it; he sights and draws rays through points a and b in line with the objects A and B as they appear from his point of observation, D. The point d of intersection of the rays will be the station D plotted.

A north and south line may be drawn upon the map, and then the user, wherever he may be in the area, if only he has in view two known points and can identify them on the map, can “find” himself. He orients the map by compass, fixes upon the map and in the manner indicated his point of observation, and may then observe the distance and direction of any other point in the area, whether visible or not.

The measurement of distance by pacing has been noted. Practice is requisite, before one can so measure distance accurately. When the greatest precision is desired, a bicycle wheel equipped with a cyclometer may be rolled over the course, or a surveyor’s chain may be used, or even a tape line.

The measured line B-C of the map begun as above described is the base line of the map. It should be carefully chosen, carefully measured, and carefully plotted; for all the rest of the map will, in accuracy, be conditioned on the accuracy with which this base line is drawn. In location it is preferably (though not necessarily) situated near the center of the area to be mapped; in length, it is best that it be about one third of the distance across the area. Its terminal points should be conspicuously marked, and widely visible throughout the area; and, for ease and accuracy of measurement, it should lie across level ground. A reach of railroad is an ideal base.

It will often be the case—generally in mountainous regions—that an adequate level base cannot be found; the terminals B and C may be eminences unequal in height, and between may lie mountain slope or valley. Now the third of the factors mentioned at the outset, elevation, has to be taken into account. It is not the surface distance between the two points B and C which is to be ascertained, nor even the distance from one point to the other on an air line, but the distance projected upon a horizontal plane—for that is what the map is intended to afford, the horizontal distance from point to point. In order to determine this distance, if the ground between be other than substantially level, the distance along the surface must be measured (keeping a straight course by compass if necessary) and the slope from point to point must be measured. To determine the angle of slope one may either use a slope board or a clinometer (an instrument built on the principle of the sextant). Having measured distance and angle of slope, one may betake himself to schoolboy trigonometry and a table of logarithms, to determine the corresponding distance in horizontal plane.