676

UNITED STATES COAST AND GEODETIC SURVEY.

(a) Locating points identifed on several photographs on the platting sheet.—The three stations A, B, and G are platted, either as parts of the triangulation System, or by measuring the base line AB on the'ground and measuring the* horizontal angles CAB, CB A, and A CB, after which the sides AC and BC may be found graphically (or by computation) and the triangle ABC may now be

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platted upon the working plan. Horizontal angles or directions to JD having also been observed from A, B, and C, its position with reference to those three points may also be platted. To plat the three picture traces HH we must know the horizontal angles P Ad ( = a), which are observed in the fleld for each picture by means of the horizontal circle attached to the phototheodolite.

The angles a are platted as aA, aB, and ac, fig. 50, and the constant focal length (=/) of the three négatives 1,4

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II, and III, fig. 51, is laid off on the radiais APa, PPB, and CPc. Perpendiculars erected to these lines in PA, PB, and Pc, respectively, will represent the oriented picture traces HAHA, HPHr, and HCHc, when the abscissæ PAdA, PBdB, and Pcdc, measured on the négatives I, II, and III, should equal the lengths PAdA, PBdB, and Pcdc on the picture traces.

The point D is termed a “reference point.” Every picture that is to be used in iconometric platting should contain the image of at least one such reference point of known position in both the horizontal and vertical sense.

After the picture traces HH hâve once been platted, any other point, T, of the terrene, shown on two or more photographs, may readily be platted from the photographs without requiring instrumental measurements in the field.

To locate the platted position of the point P, shown on two pictures, I and III, as t, the abscissæ, PAtA and Pctc? are laid off on the picture traces HAHA and HCHC, respectively, from PA and Pc and on the proper si de of P to correspond with the position of the image t with reference to the principal point, P, of the perspectives. Lines drawn from A and C through tA and tc, fig. 50, represent the lines of horizontal directions to P, and their point of intersection locates the position of P on the plat with reference to A, B, and C.

(b) Détermination of the élévations of pictured points.— The horizon line HH' of a perspective, fig. 49, being the intersection of the vertical picture plane MN with the horizon plane (passing through the optical axis of the caméra), will intersect points in the picture which in nature hâve the same élévation as the optical axis of the caméra or as the point of view S.

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