REPORT FOR 1897--PART IL APPENDIX NO. 10.

685

Another simple method, avoiding the drawing of constructive lines on the plan, is as follows:

Take a triangle of liard rubber or wood and mark off along one side the focal distance SP, fig. 50, of the print, = ab, fig. 61, and carefully notch the triangle side at b so that the center of a fine needle, marking the platted station point, will just fit into the notch. Front the print, fig. 50, take the abscissa of the pictured point a = aa = Pa' between the points of a pair of dividers, move the triangle, fig. 61, about the needle (which marks the platted station) with the left hand until ac, fig. 61, is equal to the distance aa held between the points of the dividers. The triangle is held securely in tliis position and lines are drawn along the triangle sides ab and ac. Prolong ac beyond a and check the distance ac again to be = aa. The line bc represents the horizontal direction from the platted station b to the platted reference point c (on the négative, fig. 50, the picture of the corresponding reference point is a). We will now hâve: ba = trace of the principal plane, ac = trace of the picture plane, a = projection of the principal point on the platting sheet.

The trace of the principal plane (= ab) is preferably marked on the platting sheet by a short line only, bearing an arrow pointing toward the platted station (b) whence the picture was taken, and the principal point a is marked to correspond with the désignation of the print. It may be remarked here that as few lines as possible are drawn on the platting sheet to avoid confusion and mistakes. (See photograph board.)

(d) The identification of pictured points on several photographs repre-senting identical points of the terrene.—The topographie survey being platted mainly by the intersections of horizontal directions, points eon-trolling the relief of the same area must be identified on sets of pietures taken from different stations. When selecting such points on a photograph preference should be given to those which best define the surface relief or terrene, like characteristic points of ridges, peaks, saddles, changes of slope, changes in the river courses, etc., each point being marked by a dot in red ink on the photograph and having a number or Symbol afîixed to it. It will now be necessary to identify as many of these points as possible on other photographs, covering the same area, and these are similarly marked by red dots, and identical points are given the same désignation by number or Symbol in red ink.

The identification itself of points on several pietures offers no serious difficulties, and, with some practice, as many points as may be needed for a full development of the terrene, even under different illumination of the pictured areas, may be picked out with rapidity and certainty.

(e) Application of Prof essor HaucPs method for the identification of pictured points.—In cases of doubt, when attempting to identify the same point on two different photographs, beginners may take advantage of Professor Hauck’s method, which lias been described in Chapter I, Paragraph VII.

The two photographs are pinned side by side on a drawing board. The images of the caméra stations whence the pietures were obtained are “ kernelpoints,” and if they fall outside of the picture limits they are determined from the ground plan and platted on the drawing board. The parallels to the principal lines of the photographs on which the scales are to be laid off are drawn in the mariner explained in Ghapter I and the scales are fixed in position. A fine needle is now inserted into each of the “ kernelpoints” and the loop at one end of a fine silk tliread is dropped over each needle, the other end of the thread being secured by a slender rubber band to a small paper weight (fig. 62).

A well defined point is now identified on both photographs, sufficiently far from the “kernel points,” and one thread is moved by taking the paper weight up and passing its thread, under gentle tension of the rubber band, tlirough the point just identified on the photograph, when the weight is deposited upon the drawing board, holding the thread in the given position. The same operation is repeated with the other silk thread and the other photograph, when the two threads should intersect the scales at identical division marks. If they do not, one of the scales is to be moved until both threads bisect the same division marks of the scales.

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