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

631

If the base line A Afig. 2, be laid down on paper, in reduced scale, and if the pictures MW and M1 W, fig. 3, be brougkt into the same relative positions with reference to the platted line vvhich they had at the time of their exposnre in the field, the position T of the pictured point (with reference to the platted points A and A') may be located by drawing the rays A t and'A' t'to their intersection. To locate the platted position of T the horizontal projections of the rays A t and A' t' are brought to their intersection on the platting sheet, fig. 4, which may be done by ascertaining

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1 1 i A X r ,p à

t a*

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F. g. 3

the proper positions of the lines of intersection of the picture planes with the horizontal platting plane with reference to A and A' (by “orienting” the picture traces).

The map being the orthogonal projection of the terrene in horizontal plan, the horizontal projections of the perspectives (or picture planes exposed in the vertical plane) will appear as straight lines, termed u picture traces,” fig. 4.

The correct orientation of the picture traces forms the most important part of iconometric

, ' T ''

platting, the subséquent location of picture points being accomplished by bringing the horizontal projections of the Visual rays—lines of direction—drawn to identical points to their corresponding intersections.

I. ORIENTING THE PICTURE TRACES ON THE WORKING SHEET.

(1) A base line AA', measured in the field, has been platted to scale, fig. 2, and two pictures, MW and M'W, fig. 3, had been obtained from the caméra stations A and A1 respectively by means of a surveying caméra. The focal lengths of the pictures (=/ and f respectively), the positions of the principal points (P and P1), and the horizon lines may also be given.

It is desired to locate T with reference to AA' upon the working sheet.

The distances: ÀP=f; A'P' =f (fig. 4); /P, t'P1, Pa‘ and P'a (to be measured on the pictures MW and M'W' respectively) and the line AA' are given.

The distances Aa' and A'a may be found graphically (by constructing the right-angle triangles APa' and A'P'a), or they may be computed from the équations:

Aa' = V (AP)2 + (Pa')2

A'a= V(A'P')2 + (P'af

These distances are now laid ofl* upon AA' from A and A' respectively, semicircles are described over Aa' and A'ay and two circles are drawn about A and A' with/and f respectively, as radii.

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