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682

UNITED STATES COAST AND GEODETIC SUEVEY.

carried along with the phototopographic work to locate the caméra stations in botli the horizontal and vertical sense, with reference to the primary triangulation stations already established.

The surveyor makes a rough plat of the entire triangulation (in the field), on which he locates ail the stations occupied to enable him to recognize the weak points of his work and to plan Ms operations with a thorough understanding and good assurance of success. The instrumental work in the field is done me,rely to locate the caméra stations and certain reference points (if the triangulation points are not sufficiently close together), ail topographie features being deduced from the pictures.

The caméra stations are located either by angles taken from the station to surrounding triangulation points, by resecting, or by angles observed from the latter to the signal left over the caméra station, by intersecting, or by both methods combined.

The final value of the work dépends in a great measure upon a judicious sélection of the caméra stations in order to bring the relief of the entire terrene under proper control and to be enabled to plot ail points needed for a full development of the terrene by the method of intersections of horizontal lines of direction.

Other methods for platting the topographie features and details are employed only when the method of intersections fails on account of the caméra stations not being well situated, or on account of an insufficiency of data to give the requisite number of horizontal lines of direction for a good location of points by “intersecting.”

Each caméra station should be marked by a signal of some kind before leaving it, not to be shown on the pictures, but to be observed upon with the transit or altazimuth from other stations in order to locate the correct position of the caméra station on the platting or working plan.

Frequently it will be of advantage to set the caméra up excentrically over a triangulation station in order to include certain additional parts of the landscape in the views. The position of the excentric caméra station, with reference to the triangulation point, can readily be ascer-tained, and should always be carefully recorded.

Complété panorama sets are not taken at every caméra station, it being preferred, rather, to increase the number of stations, often occupying a station to obtain a single view only, if by doing so better intersections for the iconometric location on the platting sheet of some spécial feature are obtained. Multiplicity of stations demands but a small increase in labor, either in the field, in the extra observations of directions to reference points for their location, or in the iconometric platting in the office, and enough stations should always be occupied to give a full control of the relief of the area to be surveyed.

A certain section of the terrene may bë so located that it will be a difficult matter to select more than one station whence it may be seen. In such a case the method of “ vertical intersections” may often become useful: Two or more views of such area are taken from stations at different élévations, the greater the différence in altitude between such stations the longer will the base line be, and the better are the intersections which locate the features in question, if the latter are not too far away.

* As enough plates should be exposed to cover the ground completely, the caméra stations will hâve to be distributed in such a way that ail valleys, sinks, and dépréssions, that may be represented in the scale of the map, are well controlled (i. e., seen from different caméra stations). It is évident, therefore, that the number of stations to be occupied for the phototopographic development of a certain area will dépend in a great measure upon the character of the terrene and upon the scale of the chart.

Two or three well-defined points (so-called reference points) in each panorama view (covered by one plate) are observed with the transit or altazimuth noting and recording the vertical and horizontal angles upon the outline sketch made f>r every plate exposed. Such sketches serve to identify points with far more certai'nty than a inere désignation or description of the points observed upon. The general triaugulation notes are kept in the usual manner.

Vertical angles are observed to check the position of the horizon linefon every photograph and to correct errors due to small changes in the level adjustments of the caméra that may arise during the transportation of tbe instrument over a rough trail. The horizontal angles are needed for the location of the caméra stations and for the orientation of the pictures (picture traces) on the ïfiattmg sheet for the subséquent map construction.

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