Page 69 control and signal building 2173 



low water (MLLW) for the Pacific Ocean, except in the vicinity of the Panama Canal 

 Zone where it is mean low water springs (MLWS) . 



For further information on planes of reference, see 8224. 



21 73. Vertical Daturas • 



The vertical datum for elevations, contours, and form lines for topographic surveys 

 and nautical charts is mean high water (MHW) for the United States and its insular 

 possessions and territories (see 2311), except underlined elevations of inland mountain 

 peaks on charts, which are referenced to mean sea level. 



22. TRIANGULATION 



Complete instructions and specifications fcr triangulation are contained in Special 

 Publication No. 120, Manual of First-Order Triangulation, and Special Publication 

 No. 145, Manual of Second- and Third-Order Triangulation and Traverse. Except as' 

 modified in the project instructions, these manuals shall be followed by hydrographic 

 parties engaged in triangulation. Only the briefest discussion of the subject can be 

 included in this Manual, which is only concerned with some special applications and 

 methods of triangulation particularly adapted to hydrographic surveys. 



A triangulation scheme is a continuous chain of connected triangles in figures of 

 single triangles, quadrilaterals, and polygons, beginning with a line of known length 

 and azimuth, the latitudes and longitudes of the end stations of which have been 

 previously determined. 



When the length of one line in a triangle and the interior angles are known, the 

 other two lengths can be computed. Similarly, knowing the latitudes and longitudes 

 of two stations, the azimuth between them, the lengths of the two lines to a third 

 station, and the spherical angles in the triangle (from computation of triangles), the 

 latitude and longitude of the third station and the azimuths of the other two lines can 

 be computed. 



221. General Statement 



Triangulation is seldom an end in itself; its real value is more in its usefulness for 

 other purposes than in its immediate objective, and this is particularly true of coastal 

 triangulation, which serves as control for hydrographic and topographic surveys along 

 the coast, and on the adjacent rivers and inside waters. 



To be of most value, such triangulation stations must be established with this in 

 mind and be placed where they will be as accessible and useful as possible for the control 

 of the subsequent topographic and hydrographic surveys. Wherever practicable they 

 should be placed on points, headlands, hillocks, and other places where they command 

 strategic views of the shoreline in all directions. In some localities this is easily 

 accomplished, especially where there is high land imm.ediately inshore from the coast, 

 and highways and navigable waterways paralleling the shore make transportation a 

 simple matter. Elsewhere there may be unfavorable conditions along the coast, such 

 as the swamps along the coastlines of Louisiana and South Carolina, the rugged coast- 

 lines of rocky islands, or the mountainous areas of Alaska. 



Sextant locations should not be substituted for triangulation, except in an emer- 

 gency. In general, important stations and landmarks such as lighthouses, tanks, and 

 prominent buildings should be located by triangulation or traverse. 



