226 HYDROGRAPHIC MANUAL PaGE 78 



be made of this search (see 2272), but no station shall be reported "lost" unless an 

 exhaustive search, or a change in the locality, establishes this fact beyond reasonable 

 doubt. If such a search is impracticable, a statement relative to the extent of the 

 examination shall be furnished so that all available information as to the status of the 

 station may be properly noted in the office records. 



Ephemeral objects such as flagstaffs, 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 have been verified (see 214). 



226. Station Marks 



Each new station which is to be located by triangulation shall be marked with 

 a standard bronze station-mark disk and at least two standard reference-mark disks 

 of similar material, except well-defined natural or artificial objects located by inter- 

 section and substantially constructed objects, such as lighthouses and water tanks, 

 where disks are unnecessary or marking impracticable. 



When feasible, subsurface marks shall be established at all principal scheme second- 

 order stations where there is any probability that the station mark may be disturbed. 

 The subsurface mark shall be set in accordance with paragraph c, page 39, Special 

 Publication No. 145, Manual of Second- and Third-Order Triangulation and Traverse. 

 The disk in the subsurface mark shall be stamped identically with that of the surface 

 mark. Subsurface marks are generally not required at third-order stations. 



Azimuth marks shall not be established at second- or third-order triangulation 

 stations unless the project instructions so specify. Because of the close spacing of 

 stations, an adjacent station or some natural or artificial object will usually be visible 

 and suffice for an azimuth mark. 



Infrequently, marks are set for the location of a station but, before its position has 

 been determined, the station is abandoned because the site is unsatisfactory due to 

 obstructed lines or other causes. In such cases, the marks must be removed and the 

 station destroyed. 



In selecting a site for a station, it is always important to consider both the present 

 and future use of the station. It is essential to have a location where the control station 

 is the most effective for immediate use. For the control of subsequent surveys it is 

 important to have a location and type of station mark that will remain intact and 

 undisturbed for generations. 



Triangulation stations shall be marked as permanently as possible, because, at a 

 cost of several hundred dollars per station, it is poor economy to establish a station 

 in a hasty haphazard manner so that it may soon be lost, when a little more effort and 

 care would have resulted in a more permanent station. Each disk should be secured 

 effectively in rock or concrete, to resist removal, change of elevation, or rotation. All 

 lettering shall be stamped on the disk before it is set in concrete to avoid the possibility 

 of breaking the inital set, thus loosening the disk. Where stations are established in 

 locations frequented by the general public, they should be located where they will be 

 most protected from acts of vandalism. Where they are established along the shore, 

 they should be located, if practicable, where they will be immune from destruction by 

 normal beach or riverbank erosion. Experience has proved that small terra-cotta 

 tile, filled with concrete into which a standard disk is set, is a poor material for marking 

 a station. The tile is easily broken, the core of concrete soon crumbles, and the station 

 is lost. Further specifications and particulars for marking triangulation stations may 



