Page 71 control and signal building 2214 



tions in storms, heavy rain, or strong winds, and survey parties should not be detailed 

 ashore for triangulation observations in threatening or inclement weather. Poor runs, 

 closures, and checks usually result from such observations, and it is quite probable 

 that a station occupied under such conditions would have to be reoccupied. Accuracy 

 is the first requisite of a traingulation survey and, as it is obtained, speed and progress 

 naturally follow. 



It should be realized, however, that the great accuracy of triangulation is required 

 primarily to avoid accumulations of errors in the extension of the triangulation, and 

 not to control the topographic and hydrographic surveys in a limited area. Where 

 these await the advancement of the triangulation, no opportunity should be lost in 

 expediting the observing. In such cases it may even be advisable for a time at least 

 to take preliminary .observations of a lower accuracy in order to provide control quickly. 



In regions where good visibility is infrequent and its duration short and uncertain, 

 owing to changeable conditions, low passing clouds, shifting fog patches, etc., economy 

 in observing is obtained with the use of direction theodolites. 



2214. Instruments 



Among the instruments used by the Coast and Geodetic Survey in establishing 

 triangulation control for hydrographic surveys are the 9- and 6)^-inch direction theod- 

 olites and the 7-inch repeating theodolites. The dkection theodolites most frequently 

 used were designed by D. L. Parkhurst, Chief of the Instrument Division of the Bureaii, 

 and are called Parkhurst theodolites. They are manufactured to order by reputable 

 survey instrument companies. A manufactured tripod is not used with the 9-inch 

 theodolite; the instrument is ordinarily used for observations from steel towers, but 

 for observations from the ground it is mounted on a specially built wooden stand. 



The distinctive features of this theodolite are its nonbinding centers, ball-bearing 

 clamp rings, illumination through the central axis, electrically illuminated glass 

 micrometer drums, discontinuous conical bearings for horizontal circle, and the use of 

 nylon threads for the cross hairs of the telescope and ior the micrometer wires. The 

 9-incli direction theodolties reading to single seconds on the horizontal circle are used 

 principally for first-order observations when highly accurate results are required. 

 The 6}2-inch direction theodolites reading to 2 seconds on the horizontal circle are 

 used principally for second-order triangulation and as much third-order work as prac- 

 ticable. Tripods constructed by the manufacturer are provided for the 6K-inch direc- 

 tion theodolite. 



The repeating theodolites, reading to 10 seconds on the horizontal circle, are 

 generally used for third-order triangulation when direction theodolites are not avail- 

 able or cannot be used because of the unstable condition of the ground or support of 

 the instrumen t, or the limited space available for the observer, in which instances it is 

 advantageous to be able to observe with a minimum of walking around the tripod. 



2215. Instrument Stability 



Instability of the instrument is a frequent source of error in the observations, 

 the importance of which is often not realized by an inexperienced observer. The 

 clamps and foot screws of the instrument should be kept tight at all times. If a wooden 

 stand 4 feet high is built to support the instrument, the observer should stand on foot 

 boards supported by stakes driven in the ground well away from the legs of the stand. 

 If an instrument tripod is used, it should be supported on 2- by 4- or 3- by 3-inch stakes 



