Page 63 control and signal building 2141 



was far from solved. Cement was then used to cast concrete blocks and posts for 

 survey marks, with crosses or bolts in the top defining the center. This type of mark 

 solved the problem of corrosion, but only a very limited amount of lettering could be 

 cut or molded into the top of the mark to provide future identification, and to inform 

 the public of the purpose of the mark and the importance of not moving or destroying it. 



Small metal disks were designed about the beginning of the twentieth century 

 for use as station marks. These can be set in concrete blocks or cemented in drill 

 holes in outcropping rock, boulders, etc. The legends on the marks have been changed 

 from time to time. At present, the disks of the Coast and Geodetic Survey have 

 identifying symbols denoting the type of station, and have legends giving the Bureau's 

 name as the establishing agency, explaining how to obtain information relative to the 

 stations and the penalty imposed by law for malicious destruction. The name of the 

 station and year it was established are stamped in the space provided near the center 

 of the disk. The standard disks of the Coast and Geodetic Survey. are of cast bronze, 

 3% inches in diameter, with a shank 2% inches long split at the end for spreading to 

 anchor it more securely when placed in a concrete block or drill hole. Other survey- 

 ing organizations use similar metal disks of bronze, brass, or aluminum. 



Many natural and artificial objects whose positions are determined require no 

 marking to make them recoverable. Some of these, such as sharp mountain peaks, 

 pinnacle rocks, and conspicuous boulders will very probably last for eternity. Others 

 are lone trees, beacons, water tanks, smokestacks, church spires, radio towers, light- 

 houses, etc., and will last only a few generations or less. Isolated objects are especially 

 prominent and easy to recover. 



2141. Frequency of Recoverable Stations 



In changeable areas, where resurveys are expectable because of natural or arti- 

 ficial changes, recoverable stations shall be established at 1-mile intervals along the 

 coast, conveniently located to serve as control for future topographic or hydrographic 

 surveys. 



In isolated areas, along difficult coastlines, and where natural or artificial changes 

 are not expected, recoverable stations shall be established at not less than 2-mile 

 intervals. 



2142. Descriptions of Recoverable Stations 



All triangulation stations shall be described on Form 525, Description of Triangula- 

 tion Station, in accordance with 227. All recoverable topographic and hydrographic 

 stations shall be described on Form 524, Description of Recoverable Hydrographic or 

 Topographic Station, in accordance with 2351. The detailed descriptions of well- 

 defined natural or artificial objects may be brief but must be thorough and made with 

 care, to avoid confusion or doubt about their recovery at some future date. 



The descriptions of structures, such as lighthouses, flagpoles, and water tanks 

 are particularly important. These are subject to removal and replacement in adja- 

 cent locations, where the new structure may be mistaken for the former one unless the 

 circumstances are known. Errors may thus be introduced in the survey which may 

 be difficult to detect and, when detected, the source of the error may be very difficult 

 to determine. Descriptions of lighthouses shall include the dates of their establish- 

 ment, their heights, and brief descriptions of their appearance. Descriptions of water 

 tanks must be made with even more care, and must include the names of the companies 

 or organizations owning them; their locations, with measured distances to adjacent 



