A Treatise on Levelling, Topography and Higher Surveying. (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1870 Excerpt: ...method is a combination of the first, second, and third systems. OHAPTEE I. FIRST SYSTEM. BY HORIZONTAL CONTOUR-LINES, (133.) General Ideas. Imagine a hill to be sliced off by a number of equidistant horizontal planes, and their intersections with it to be drawn as they would be seen from above, or horizontally projected on the map, as in Fig, 96. These are "contour-lines." Fig. 96. They are the same lines as would be formed by water surrounding the hill, and rising one foot at a time (or any other height), till it reached the top of the hill. The edge of the water, or its shore, at each successive rise, would be one of these horizontal contour-lines. It is plain that their nearness or distance on the map would indicate the steepness or gentleness of the slopes. A right cone would thus be represented by a series of concentric circles, as in Fig. 97; an oblique cone, by circles not concentric, but nearer to each other on the steep side than on the other, as in Fig. 98; and by a half-egg, somewhat as in Fig. 99. (134.) Plane of Reference. The horizontal plane on which the contour-lines are projected, and to which they are referred, is called the "plane of reference." This plane may be assumed in any position, and the distance of the contour-lines above or below it is noted on them. It is usually best to assume the position of the plane of reference lower than any point to be represented; so that all the contour-lines will be above it, and none of them have minus signs. (See Art. 59.) (135.) Vertical Distances of the Horizontal Sections. These depend on the object of the survey, the population of the country, the irregularity of the surface, and the scale of the map. In mountainous districts they may be 100 feet apart. On the United States ...