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

691

was taken off the “scale of heights,” is set in <7, flg. 69, and the division mark D of the scale, coinciding with the other point of the dividers, will indicate the height of the point above the plane of reference or datum plane.

This height is entered in pencil on the plan, inclosed in a small circle, to distinguish it from the number of the station. It is checked by means of a second photograph, and when the discrepancy between the two values for the élévation of the point is within the permissible limits of error, their mean value is entered in red ink on the plan and ail pencil figures are erased.

Any marked différence in the value for the height of a point obtained from two photographs would indicate either that the two points selected on the photographs do not represent the same point of the terrene, or that an error in platting or in finding the height had been made. A third intersecting line from a third station wonld dispose of the first two alternatives, and a new measurement of the height will show whether an error had been made, or whether the discrepancy is due to unavoidable errors.

(j) The use of the so-called uphotograph board.”—The various constructions described in the preceding pages, if made directly on the platting sheet and on the photographs, would produce confusion in the iconometric platting, owing to the intricacy of the lines, and would obscure many details in the pictures. Captain Deville, therefore, has h ad a spécial drawing board prepared on which as many of the construction lines are drawn, once for ail, as would hâve to be repeated for the different prints of uni-form size and which had been obtained with the same caméra.

This so-called “photograph board” is an ordinary drawing board, covered with tough drawing paper, the surface of which is to represent alterna-tively either the picture plane or the principal plane (both revolved into the horizon plane). It is used in conjunc-tion with the photographs or négatives.

Two lines, DD' and SS', fig.

70, are drawn at right angles to each other. They represent the horizon and principal lines, while PD = PD1 = PS' =/= focal length, so that D, D', S, and S1 are the left, right, lower, and upper distance points, respectively.

The photograph is placed in the center of the board, the principal line coinciding with SS/ and the horizon line with DD1, in which position ( TYZO) it is secured to the board by means of small thumb tacks or pins. The four scales, forming the sides of the square OTYZ, serve, among other purposes, to locate lines parallel either with SS' or DD1 (without actually drawing the parallels) on the photograph, the latter falling within the limits of the square OTYZ. At a suitable distance from the distance point D' a perpendicular QR is drawn, on which are marked by means of a table of tangents the angles formed with DQ by lines drawn from D. This scale may be used for measuriug the altitudes or azimuthal angles of points of the photograph, as will be explained in a separate paragraph later.

From S as a center with SP =/ = focal length an arc of a circle PL is described and divided into equal parts. Through these points of division, and between PL and PD', lines are drawn converging to S. Parallels MN to the principal line are also drawn, as shown in fig. 70. Ail these lines are used in connection with the scale of degrees and minutes QR.

FiG.70

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