Direction is fundamental. Suppose there are three dominant points in the area to be mapped, relatively situated as here indicated.
The first problem is, to get those points set down on paper accurately, in proper relative positions.
The map-maker begins, say, at B. He has provided himself with a sketching board, having a sheet of paper tacked upon it, and with a ruler and a pencil. He sets his board up and carefully levels it. He then marks upon the paper a point b which in the completed map is to indicate this station B of first observation—the point where he now stands. Knowing in a general way the area which he wishes to map, and observing from his station the directions in which the distant objects A and C lie, he so places point b that his paper will afford space for the intended map.
The map-maker then lays his ruler upon the paper, brings its edge close to point b, and sighting from point b on the paper to the distant object A, turns the ruler until its edge coincides with the line of sight. Then he draws upon the paper a line or “ray” from point b toward object A. In like manner he sets his ruler again and draws a second ray, from b toward the distant object C, thus:
Having fixed point b and drawn the two rays b-A and b-C, the map-maker leaves station B and goes to either of the other points: to point C, say. He there sets his board up again, and levels it carefully as before. He turns the board until, sighting along the previously drawn ray C-b, the now distant station B is exactly covered. Then he lays the ruler again upon the paper, and turns it until, sighting along its edge, distant object A is exactly covered. He then draws a ray along the edge of the ruler thus:
The points a and c, where this ray intersects the two previously drawn rays, are the presentment of the points A and C in the area under observation, and a map of the area is begun.
These three points may be mountain summits, trees, telegraph poles, chimneys, or any other conspicuous features of the landscape, and they may be distant one from another 50 miles or 500 yards; they are set down on paper in their true relative positions; they are mapped.