2216 



HYDROGRAPHIC MANUAL 



Page 72 



firmly driven into the ground. Wliere tundra or heavy moss is encountered, the roots 

 should be cut around each tripod leg so the pressure from walking around or standing 

 near the tripod legs will not disturb the azimuth of the instrument during observations. 

 The instrument and, if practicable, the stand or tripod, should always be shaded 

 from strong sunlight. 



2216. Siim Angles 



Sum angles must not be observed with a repeating theodolite. When additional 

 lines must be observed from a station previously occupied, the new observations must 

 include one and only one of the previously observed lines, except when check angles 

 are observed to prove the recovery of old stations (see 225). 



For the observing program illustrated in a in figure 5, it is assumed that the observer at A observes the angles DAE, EAF, FAB 

 and, being unable to see station C, closes the horizon by observing the angle BAD, as shown by solid hues and arrows. At a later 

 date the observer returns to A when C is visible and correctly supplements his former observations by observing the angles BAC 

 and CAB as shown in 6 in figure 5. 



AC 



AC 



AB 



a b c 



Figure 5. — Additional lines observed with repeating theodolite. (The correct procedure is illustrated in a and b; the incorrect in a and c. 



The wrong method of observation is illustrated in e in figure 5. The angles BAC, CAD, and DAB must not be observed since 

 they include two previously observed lines AB and AD. This results in a sum angle, since the sum of the angles BAC and CAD 

 is equivalent to the previously measured angle BAD and will require a least square adjustment to distribute the difference in values 

 properly. If the observations are made as stated in the previous paragraph, the angle CAD may be obtained by subtracting the angle 

 B-4Cfrom the previously obtained value of the angle BAD. 



Wlien a direction theodolite is used in second- or third-order triangulation, the 

 horizon is not to be closed by repointing on the initial. This results in a second value 

 for the initial which must then be meaned with the first mean and is entirely unneces- 

 sary. Such observations are made only occasionally to test the plate for drag. (See 

 figures 6 and 7 for methods of recording observations by direction and repeating 

 theodolites.) 



222. Reconnaissance 



The project instructions will specify the order and types of control to be estab- 

 lished, the connections to existing stations, and the astronomic stations to be occupied 

 or the bases to be measured. Special Publication No. 225, Manual of Reconnaissance 

 for Triangulation, describes methods of ground reconnaissance for triangulation, com- 

 putations for intervisibility of stations, strength of figures, etc. 



