2281 HYDROGRAPHIC MANUAL PaGE 84 



accurate positions may be determined in such cases and often a situation, which at first 

 appears almost insurmountable, may be satisfactorily solved. Generally, stations 

 located by such methods must be considered topographic stations, and marked and 

 described as such, for although the methods often more nearly resemble geodetic 

 methods, usually some phase does not meet one of the requirements for third-order 

 triangulation (see 211). The few typical substitute methods described here are only 

 illustrative of what may be accomplished in this manner. 



2281. Location by One Angle and Range 



A station is often needed in a locality where its position may be readily and quickly 

 determined by placing it on range with two existing stations and occupying it to 

 measure the angle between the range and a third existing station. 



In figure 8, the old stations MARION, RUDOLF, and DEVON have been previously recovered and signals have been built 

 over them. A station on an island is needed quickly for the control of the topography, when it is discovered that the limits of the 

 topographic sheet are such that a satisfactory topographic position cannot be obtained. 



]"~^r:2^ DEVON 



Figure 8. — Location by range and one observed angle. 



Investigation discloses that a new station ALBINO can be conveniently located on range with RUDOLF and MARION at a 

 point from which DEVON can be seen. This is accomplished by setting up a theodolite on range by eye, and then shifting it slightly 

 until the signal poles at the two stations are precisely on range when viewed through the telescope, their verticality being checked 

 at the same time. 



The angle at ALBINO between the range line and station DEVON is measured, this being the only measurement necessary to 

 determme the position of station ALBINO. The angle at RUDOLF is obtained from the forward azimuth of the line to DEVON 

 and the back azimuth of the line to MARION, and the angle at DEVON is concluded to complete the triangle. The position of 

 ALBINO can be computed from the triangle ALBINO— DEVON— RUDOLF or the triangle ALBINO— DEVON— MARION. 



If the three old stations have been indisputably recovered (their reference marks 

 having also been recovered and the reference distances checked), no additional check 

 angles need be observed. In this case there is no check on the position, and a station so 

 determined shall be marked with a standard topographic station-mark disk and 

 described on Form 524, Description of Recoverable Hydrographic or Topographic 

 Stations. It is not to be considered a triangulation station since the method does not 

 meet the requirements for third-order triangulation. 



If all three recovered triangulation stations had come within the limits of the 

 topographic sheet, a station could have been located graphically by setting up the 

 planetable on the range and resecting on the third station. 



