2111 HYDROGRAPHIC MANUAL PaGB 60 



eluded; (6) intersection stations located by an insufficient number of directions to 

 provide an adequate check of their positions; (c) stations located by theodolite by the 

 three-point problem, without a check; {d) temporary stations, located by geodetic 

 methods, but which are unmarked and nonrccoverable; and {e) stations located by any 

 method or scheme, which depends in part on floating or movable stations. The last 

 classification applies particularly to stations located by the ship- and buoy-intersection 

 methods, described in 2282 and 2283, or similar methods. 



Distinction is made between topographic and hydrographic stations according to 

 the methods of location. 



2111. Stations and Signals 



A station is a definite point on the surface of the earth whose geographic position 

 has been determined for control purposes. 



The term signal denotes the object, existent or especially erected, which indicates 

 the location of the station, and is used as a target" toward which survey instruments 

 may be pointed. 



Stations may be marked points (permanent or temporary) or they may be con- 

 spicuous artificial or natural objects. If the latter, the station is the center of the 

 object which is the signal. 



2112. Triangulation Station 



A triangulation station is a recoverable point on the surface of the earth, whose 

 geographic position has been determined by angular methods with geodetic instru- 

 ments. It must be located with first-, second-, or third-order accuracy (see pp. 1 to 5, 

 Special Publication No. 145). 



A triangulation station is a selected point, wliich has been marked — lately always 

 with a standard station mark — or it is a conspicuous natural or artificial object. 



Descriptions are written of all triangulation stations. 



2113. Traverse Station 



A traverse station differs from a triangulation station only in that its position is 

 determined from another known point by linear measurements controlled in azimuth 

 by angular measurements, instead of by angular methods solely. 



2114. Topographic Station 



A topographic station is a definite point on the surface of the earth, whose geo- 

 graphic position has been determined by graphic methods, usually by planetable 

 traverse or graphic triangulation. 



Classed also as topographic stations are those points located by geodetic methods 

 but with less than third-order accuracy, as mentioned in 211. 



Points located by the radial-line plot of an air photographic survej^, specifically 

 for use in a hydrographic survey, are also classed as topographic stations, as well as 

 all stations that are symbolized by a red circle in accordance with 2393. 



A topographic station may be recoverable, if it is marked by a standard station 

 mark or is a conspicuous natural or artificial object; or it may be temporary, usually 

 for use in the subsequent hydrographic survey. 



Descriptions are written of recoverable topographic stations. 



