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

683

(b) General remaries on the iconometric platting of the survey.—The field notes of the phototopo-graphic survey s made in the Northwest Territory of the Dominion of Canada by the topographical surveys branch of the department of the interior (under Capt. E. Deville, surveyor of Dominion lands), are platted on a scale of 1:20000, but the maps are published on a scale of 1:40000, with (équidistant) contours of 100 feet vertical intervals.

The phototopographic reconnaissance in southeastern Alaska, executed by Dominion land surveyors under Dr. W. F. King, Alaskan boundary commissioner to Her Majesty, was platted on a scale of 1:80000 and published on a scale of 1:160000, with horizontal contours of 250 feet vertical intervals.

It is assumed that the triangulation computations hâve been made and that the triangulation points hâve” ail been platted, and that their élévations above the adopted reference plane hâve been affixed to the marked points on the platting sheet.

The triangle sides of th,e secondary triangulation scheme (executed during the phototopographic survey) are now computed (the corrections to the horizontal angles, indicated by the closing errors, haying been applied), the latitudes and departures (from every secondary point to the nearest primary station) are computed, and the secondary stations are then platted by their latitudes and departures (unless the primary triangulation sides are too long).

The caméra stations (not included in the secondary triangulation scheme) are now platted with reference to the triangulation points, using a table of chords or a station pointer (three-arm vernier protractor). If many points had been observed upon from the caméra station, the horizontal angles are preferably laid oft* on a piece of tracing paper, and this improvised multi-arm protractor is used like a station pointer to locate the station.

The surveyor should endeavor to obtain at least one direction from a triangulation station to every caméra station; the (iconometric) platting will then be less troublesome and more accurate.

Photographs should not be used for platting the positions of caméra stations, as this would not locate them with sufficient précision, and enough angles should always be observed in the field to locate every occupied station in the manner just mentioned.

From the original négatives copies are made, enlarged to 9£ by 13 inches on heavy bromide paper, more recently, however, a spécial brand of bromide paper, known as “platino-bromide,” has been used by Captain Deville. The enlargement adopted in Canada for these bromide prints is about 2.1 times larger than the négatives, which ratio was selected to utilize the full width of the paper found in market.

These bromide enlargements are used extensively in the map construction, and they should be made with great care to reduce distortion to a minimum. Before using the prints for the map construction any distortion due to the enlarging process should be ascertained, which is done in the foliowing manner :

Fs g. 58

Join the middle notches H and H', indicating the position of the horizon line, and P and P7 represeuting the position of the principal line, fig. 58, and with a set square test these two lines for perpendicularity. Take with a pair of dividers the distance between the two notches A and B (which on the négative is equal to one-half of the constant focal length) and see whether it is/

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