4. CONTROL AND SIGNAL BUILDING 



113 



shall be made on Form 526, Recovery Note, 

 Triangulation Station, for each station re- 

 covered or searched for. A station should 

 not be reported as lost unless there is con- 

 clusive evidence to establish the fact beyond 

 a reasonable doubt. The description of the 

 station shall be verified in detail or corrected 

 as necessary to conform with circumstances 

 at the time of recovery. Damaged stations 

 or reference marks should be repaired. If it 

 is necessary to move any of the marks at a 

 station, the procedures described in Serial 

 632, The Preservation of Triangulation Sta- 

 tion Marks, shall be used. All triangulation 

 stations shall be marked or re-marked and 

 the disks stamped in accordance with rules 

 stated in Special Publication No. 247. 



Where new triangulation is connected to 

 previously established triangulation, it is 

 necessary to verify recovery of the old sta- 

 tions, and to check the distances to reference 

 marks and the angle between them to ensure 

 that none of the station marks have been 

 moved. This will ordinarily be considered 

 adequate proof of recovery of a third-order 

 station. When the connection is made to 

 first or second-order triangulation, the two 

 stations at the ends of the old line must be 

 occupied and observations made to a third 

 station of the original scheme. The measured 

 angles should check the original observations 

 within 3 seconds as proof of recovery. 



Objects such as flagpoles, chimneys, smoke 

 stacks, beacons, and signal towers, whose 

 positions have been previously determined, 

 shall not be used for control until they have 

 been positively identified or their original 

 positions verified. 



4-5 Station marks and descriptions. — 



Each new station which is to be located by 

 triangulation shall be marked with a stand- 

 ard bronze station-mark disk and two stand- 

 ard reference mark disks, except well defined 

 natural or artificial objects located by in- 

 tersection and substantially constructed ob- 

 jects, such as lighthouses and water tanks, 

 where disks are unnecessary or marking 

 impracticable. Subsurface marks shall be 

 established at main scheme second-order sta- 

 tions where practicable. Azimuth marks shall 



be established at second- and third-order sta- 

 tions unless another triangulation station or 

 at least two objects such as lighthouses, 

 tanks, church spires, etc., are visible from 

 the ground at the station. Instructions for 

 naming and marking stations, and for stamp- 

 ing the disks are contained in Special Pub- 

 lication No. 247. 



Each new marked triangulation station 

 shall be described on Form 525. The descrip- 

 tion should be clear, concise, and complete. 

 It should enable one to go with certainty to 

 the immediate vicinity of the mark, and by 

 the measured distances to permanent refer- 

 ence points it should inform the searcher of 

 the exact location of the station. When 

 marked stations of other organizations are 

 included in the triangulation scheme, they 

 should be described on Form 525. 



All objects, such as spires, tanks, etc., lo- 

 cated by intersection, shall be described on 

 Form 525-b. An observer shall visit the sta- 

 tion and identify the object by a descriptive 

 name, name of owner, and year of construc- 

 tion. Lighthouses and other fixed aids shall 

 be identified by name as shown in the most 

 recent issue of the Light List. 



4-6 Connections with triangulation of 

 other organizations. — Independent schemes 

 of triangulation, which have been established 

 in the project area by other organizations, 

 shall be connected to the triangulation of 

 this Bureau by strong figures in such a way 

 that their positions may be computed and 

 adjusted on the datum appropriate to the 

 area, (NA-1927 in the continental U.S. and 

 Alaska) if this is feasible. A line connection 

 is preferable to a point connection. Federal, 

 State, and local agencies should be contacted 

 to ascertain what control exists in the proj- 

 ect area, and to obtain copies of descriptions 

 and positions of marks and diagrams of the 

 control schemes. 



If practicable, the positions of stations 

 established by other organizations should be 

 determined in lieu of establishing new sta- 

 tions nearby, provided the station marks are 

 in good condition and suitably located. Two 

 standard reference marks should be estab- 

 lished if none exist. Under no circumstances 



