696

UNITED STATES COAST AND GEODETIC SURVEY.

Should the mountain bave rounded forms and no well-defined ridge, the visible outline on tbe photograph may be assumed to be contained in a vertical plane perpendicular to the line of direction drawn to the middle of the ridge outline.

The construction, fig. 74, is made by drawing the line of direction SM to the middle of the ridge outline and S V perpendicular to SM. On the plan px Jfi is made = PM, and from the projection a of the summit a of the mountain a perpendicular ac to Sx Mx is drawn, which will represent the horizontal projection (ac) of the pictured outline (ay) ; it is produced to the intersection X with the picture trace Xx Yx. PQ is taken (on the photograph) equal topiX (on the plan), and the scale of équidistances is placed at Q perpendicular to the horizon line PD'. The division mark at Q corresponds with the élévation of the station S, and the points of division corresponding with the contours are joined to V and produced to their intersections with the outline a . . y. The platting is done as in the preceding case, or the lines of direction drawn to the points of intersection of the outline ay by the contour planes may simply be platted and the contour lines on the plan may be drawn tangent to these lines of direction.

The horizon line, containing the perspectives of ail points having the same élévation as the

station, represents the perspec-Fig. 75 tive of a contour line when

the caméra horizon is identical with a contour plane. The iconometric draftsman should pay particular attention to géologie forms and to the origin of topographie features, as with-out such applied knowledge a correct interprétation of such forms and their cartographie représentation would require the cartographie location of a vast number of control points to obtain a faithful représentation of the terrene forms. Although the latter may often resuit from the successive or combined actions of manyagen-cies, they will yet hâve similar récurrent characteristic shapes when produced by the same causes, and the contours, being the means for delineating the cartographie représentation of the terrene shaped by identical agencies, should also show a corresponding characteristic similarity.

(m) The photograph protractor.—The angle included between the line of direction (to a point of a photograph) and the horizon, or the principal plane—the vertical or altitude and the horizontal or azimuth angle—is sometimes wanted.

The horizontal angle may be obtained directly on the photograph board by joining the station S, fig. 75, and the projection a (on the horizon line) of the pictured point a. If required in arc measure, the distance Pa may be transferred to the principal line SS' from P = PG; D is joined to G and produced to the scale of degrees and minutes BG, where the graduation mark Je indi-cates the value of the horizontal angle in arc measure.

Whenmany such angles are tobemeasured, thehorizontal scales TY and OZ, fig. 75, may be divided into degrees and minutes by means of a table of tangents, using as radius the focal lengtli SP.

The altitude is the vertical angle at S of the right-angle triangle, having for sides Sa and aa. To construct it, take DF = Sa, draw FF parallel witli and equal to aa, join’D and F, and produce DF to the scale B G of degrees and minutes.

This construction wrill be facilitated by the lines already drawn on the photograph board, fig. 70. With a pair of dividers take the distance (abscissa) from the pictured point a to the



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