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

681

FlCr 57

\

c'OSll = a' represents the horizontal angle inclucled between the principal plane and base line

I II, and c" O Si = a" represents the corresponding horizontal angle for station II. These angles a' and a" are transferred from the négatives I and II to the corresponding ends of the base line III, as indicated in fig. 56.

After laying ofï the focal length/from the base stations I and II npon the sides of the angles a‘ and a" ( = le' and Ile" respectively) and erecting perpendieulars {If'H1 and H"H") in c1 and c" to le’ and Ile" respectively, they will represent the oriented picture traces of négatives I and II.

The remaining two sets, of five plates each, of the panorama view s at the stations I and II, are easily oriented and platted, the next plate in order at station I, for instance, will hâve the I y.

optical axis" in the direction a' + 60°, the third : a' + 120°, etc.

After ail the horizontal projections of the ver-tical plates (picture traces) -Si-Hi, ....

HeHc, fig. 57, hâve been platted at both stations I and II, the horizontal projections of ail points that may be identified on two plates are marked and platted by locating the intersections of the lines of direction drawn througk the projections on the picture traces of the pictured points in the same manner as shown in fig. 56 for the tower T. Every platted caméra station will be surrounded by a regular hexagon formed by the picture traces of the six plates comprising the panorama set.

(c) Détermination of the élévations of pictured pomts of the terrene.—The projection in horizontal plan of an object having been platted, the élévation I(Si) of that object S, above (or the dépréssion of it below) the horizon, HH, of the caméra station

II may be found as follows :

The lengths IItfx(= 0$ on PI. II) and III, fig.

56, may be measured on the platting sheet, and the ordinate ya may be taken from the négative II.

We erect perpendieulars to III in St = ys = St (SL) and in I, then draw the line II(Si) to its intersection (Si) witli the perpendicular to I II in I, when the length 1(8;), measured in the platting scale, will represent the différence in élévation between the points I and II.

By computation we would find from : '

I(Si) :ya = III: StII

m)

Vu

III

8,11

If

the scale of the map is

_1

M’

we will hâve :

I(Si) = M.ys

III * Si II

The values of ya, III, and Sill are found by direct measurements with a small ivory scale divided into 0*5 mm., of which 0*1 mm. may be estimated after a little practice.

(3) Method of Capt. D. Deville (Ganadian method).—This*so-called Canadian metliod has been in use under the auspices of the department of the interior of the Dominion of Canada since 1888. Deville has given a full account of these methods in Photographie Surveying, published at the government printing bureau, at Ottawa, in 1895, and the following paragraphs hâve been largely taken from Deville’s book :

(a) General remaries on fiéld worlc.—The area to be surveyed is covered with a triangulation net, preferably before the phototopographic survey is commenced, and a secondary triangulation is

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