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

687

Fig. 63

When a station, A, fig. 64, falls so close to tlie edge of tlie working board that the trace QB (of the picture plane) falls outside of the limits of the plan, then the trace AG of the principal plane is prodnced to jB, making AB = AC = focal length of the pictnre, and MN is drawn perpendicular to B G or parallel to QB. The line MN will, with reference to QB, occupy the same position as the focal plane of the caméra does to the picture plane of the perspective. The direction of a point of the photograph projected in Q on the picture trace is fouud by joining N'A and producing to the opposite side of A.

As mentioned before, the intersection of the first two lines of direction should be checked either by a third line or other-wise before the position on the plan of a pictured point should be accepted as correct. Such intersections may, for instance, be checked by determining the height of the point from both photo-graphs. Unless correctly platted and correctly identified, the two values for its height will not agréé. This check, however, does not guard against slight errors in platting. A check may also be obtained by drawing a line, on which the point is situated, with the perspectograph or perspectometer, but the best check will al-ways be a third intersecting line of direction from a third station.

(g) Platting pictured points iconometrically by vertical intersections.—We had seen how the base line between two stations is projected into horizontal plan for the method of horizontal intersections hitherto considered, but when two caméra stations are occupied at different élévations (and close together horizontally) to locate features of the terrene by intersections, the so-called “method of vertical intersections” is employed. With this method the base line (its horizontal projection being either too short or more frequently falling into the direction in which the points to be located iconometrically are situated) is projected upon a vertical plane. The greater the différence in élévation between the two stations, the greater the length of this base-line projection in vertical plane, and also the better the location of the points by vertical

intersections will be.

We will hâve with reference to fig. 65:

A and B=positions of the two caméra stations, platted upon the working sheet. (A is more elevated than B). aB = horizontal projection of the base line AB. AN and Bn = two négatives (showing the images dA and dB of the same point D) exposed at the stations A and B respectively. HABHAB' and HBHB' = picture traces of the two négatives on the ground plane or working sheet. aPA' = BPQ = focal length of the négatives AN and _BN.

We will assume that the horizontal plane passing through the lower station (B) is the ground or platting plane, and the principal plane of the négative A may be taken as the vertical plane of projection. PL^H^ will then be the trace of the picture plane AN on the ground plane.

Furthermore, the principal plane, of which aPA' is the trace in the ground plane, is supposed to be revolved about aPA' into the ground or platting plane in order to simplify the construction.

To plat the position in the ground plane of a point JD, pictured on AN and BN as dA and dB respectively, the rays AdA and BdB are projected upon the vertical plane (revolved about aPA‘ into the ground plane) when (dQ, in fig. 65, will represent their point of intersection d, projected into the vertical plane = dh and revolved about aPA' into the platting plane = (dx).

The ray AdA = AD intersects or pénétrâtes the picture plane AN at a distance = dAdAB vertically above dA', on its picture trace HABHAB' (ground line of picture AN). This ordinate is laid ofif upon PAHAB = PA (dA), when (dA) will be the projection on the vertical plane of the pictured point dA.

The vertical through a projected upon the vertical plane is represented as a (A), and if we make a {A) — PAPAB (of picture AN) = différence in élévation between the two stations A and B,

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