706

UNITED STATES COAST AND GEODETIC SURVEY.

water. These boxes are shipped to the head office in Ottawa, wliere tbe plates are developed by a specialist.

The data obtained with the aid of the transit for triangulation purposes are recorded in the field book in the usual manner, as customary for such work.

The horizontal angles observed with the transit (or altazimuth instrument) to the points of the terrene marked on the outline sketch which accompanies each négative, serve not only for the orientation of the horizontal projection of the plate on the plan (the so-called “picture trace”), but they also serve to counteract in a measure and to ascertain the distortion of the paper prints (or photographie enlargeinents). The vertical angles, together with the platted distances, are used to check and verify the position of the horizon line on the different photographs.

The most important caméra stations are occupied by the surveyor ; the secondary stations by

the assistant surveyor, with his own caméra. ISTo trigonométrie observations are made by the assistant while occupy-ing the secondary stations.

Ail views are taken with the same stop : //36.

(4) The United States Goast and Geodetie Survey caméra.— The original type of the Coast and Geodetie Survey caméra, used in connection with the Alaskan boundary survey, was similar in form to Deville’s original caméra, except that it had a spécial tripod with bail and socket adjustment and that the teeth which serve to mark the principal and horizon lines on the négative could be turned by revolving one button to be pressed into contact with the photographie plate.

This caméra was also provided with a ground glass, enabling the surveyor to inspect the entire field controlled by each plate before exposure, and giving ready means for test-iug the positions of the teeth which mark the horizon line.

The caméra itself was a plain rectangular box made of well-seasoned mahogany 6f by 5| by 9^ inches in size, and it was used always in the same position, with the short faces vertical. The bamboo tripod legs were composed of three pièces, each 16 inches long, and screwed together at the joints. When dismembered the tripod was carried in a sole-leather packing case together with the caméra, twelve plates (in six double plate holders), notebook, barometer, ther-mometer, yellow color screen, etc.

The new phototopographic caméra of the Ooast and Geodetie Survey is a phototheodolite, resembling Colonel Laussedat’s latest pattern which will be described in the following pages.

IV. SURVEYING CAMERAS COMBINED WITH GEODETIC INSTRUMENTS.

(Phototheodolites, photographie plane tables, etc.)

The data acquired in the field with photogrammeters of the class just described had to be supplemented with observations made in the field with some geodetie instrument (transit, plane table, etc.) in order to obtain complété topographie survey s of the régions traversed by the phototopographic surveying party.

The idea of combining surveying instruments with a photographie caméra into single compact and serviceable instruments originated very early with phototopographic workers, and refined phototheodolites and photographie plane tables are to this day the favorite phototopographic instruments in Europe, whence they are also exported to other countries.

These more or less complicated instruments bave been devised to secure great précision in the work undertaken with them, and refined methods are employed for the field observations, for the culling of data from the photographie perspectives, and for the computations made in the office to increase the general précision of data derived from the operations executed in the field.

Generally speaking, the best results for topographie purposes are obtained by means of photography, if we bear in mind that phototopography essentially and primarily is a constructive and graphie art, based upon graphie or pictoiial records (which are nothing more t-han central

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