690

UNITED STATES COAST AND GEODETIC SURVEY.

photograph, and Bdi' = f + JW; where: Pdx' = vertical distance between the platted point dx and tlie picture trace HRHf, to be measured on the platting sheet, its value ( =E) may be found mechanically with aid of an ordinary sector, fig. 67, as follows :

Take with a pair of dividers the (ordinate) distance froin the pictured point dB to the horizon line (on the photograph), place one point of the dividers on the division G of the sector, fig. 67,

Fig.68

where OC = focal length of the photograph, and open the arms of the sector until the second point of the dividers coincides with the corresponding division I) of the other arm of the sector (OE being equal to OC = focal length), now add the length dx'P, fig. 66 (horizontal distance of the platted point dx to the picture trace EBEB projected into vertical plane), to the focal length =/ = OG, fig. 67, by placing one point of the dividers in G, when the other point may coincide with the division A of the scale O G. Hold the sector nnchanged, turn the dividers about the point A, and bring the second point to the graduation mark B of scale OE, B corresponding to A, or O B = OA ; when AB will represent the height H of the point d above the horizon plane of the station P, to be measured on the platting scale.

(i) Iconometric détermination of élévations by means of the so-called, u scale of heightsP—Another method consists in making use of the “scale of heights,” fig. 68. Make SP=f = focal length of the perspective, erect PA perpendicular to SP in P, and divide both fines into equal parts. Draw radiais from S through the points of division on PA, and through those of SP draw parallels to PA. Now, with a pair of dividers take from the photograph the distance from the pictured point to the horizon fine (the ordinate of the pictured point eorre-B sponding to P{d'h) = h, in fig. 66) and transfer it to PA from P = Pfx. The position of \x may be found to correspond to the line Sjx, passing through the point 9 of the graduation on PA.

With a pair of dividers take now (from the platting sheet) the. vertical distance from the horizontal projection of the point to the picture trace (= ôdx in fig. 66) and transfer it to the right or left of P according as the point of the plan falls beyond the picture trace or between the platted station and the picture trace. In fig. 68 it is shown as falling between the station and the picture trace into m. The line mB, drawn parallel with PA, is intersected by the radial S/.i (corresponding to scale division mark 9) in M. The distance mM, measured on the platting scale, will be the height of the point above (or below) the station. ’

A scale, fig. 69, is conveniently pinned somewhere, perpendicularly to a fine AB, the division G of the scale, corresponding to AB, being the height of the caméra station. One point of a pair of dividers with which the length AB was taken off the “ sector,” or with which the length mM

Fie. 69

e c

—M

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