6. THE SMOOTH SHEET 



235 



graphic and photogrammetric surveys are 

 deficient in these respects. 



It will frequently be found that rocks 

 shown on a prior hydrographic or topo- 

 graphic survey have not been verified by the 

 new survey or have been located in a slightly 

 different position, or are of a somewhat dif- 

 ferent character. In the disposition of such 

 cases all available information should be 

 consulted and the following rules should be 

 followed : 



(a) Where the position of what is pre- 

 sumably the same rock differs on the new 

 and the prior survey, the new position should 

 be accepted as correct. 



(b) Where an adequate examination, 

 made by the new hydrographic or topo- 

 graphic survey in the vicinity of a rock or 

 rocks, fails to disclose the existence as shown 

 on the prior survey, the recommendation 

 made by the hydrographer regarding its dis- 

 position, should be followed. 



General statements in a descriptive report, 

 particularly of a topographic survey, that 

 certain rocks on old surveys could not be 

 found should be accepted only as proof that 

 such rocks are not bare rocks. They should 

 be transferred either as rocks awash or as 

 sunken rocks, depending on the circumstan- 

 ces in each particular case. 



(c) Bare rocks on a prior topographic or 

 hydro^aphic survey, that are not shown or 

 disproved on the new hydrographic or topo- 

 graphic survey should be carried forward as 

 rocks awash. 



(d) Rocks awash on a prior survey that 

 are shown as sunken rocks on the new sur- 

 vey should be considered as rocks awash un- 

 less there is information on the new survey 

 to show that the rock was not visible at low 

 tide. In such cases the rock awash symbol 

 should be shown in black ink on the new 

 hydrographic survey and a note made in the 

 sounding record. 



(e) Sunken rocks on prior surveys, if not 

 disproved by the new survey, should be car- 

 ried forward with caution. On some prior 

 surveys a sunken rock symbol was used to 

 indicate a rocky bottom area and not neces- 



sarily individual submerged rocks dangerous 

 to navigation. 



(f) In general, the delineation inside the 

 low-water line should be accepted as correct 

 on the new survey, except that isolated rocks 

 awash shown on a prior topographic survey, 

 that are not located or disproved on the new 

 survey, should be carried forward in color. 



6-102 Geographic datums. — It is essen- 

 tial that the reviewer shall correctly identify 

 the geographic datum to which a prior sur- 

 vey was referenced and make adjustments 

 where necessary to permit comparison at the 

 datum used in the most recent survey. 



During the early years of the Bureau's 

 operations many detached triangulation sys- 

 tems existed in the United States, each 

 based on independent astronomic observa- 

 tions within the system. Hydrographic and 

 topographic surveys within the areas were 

 consequently based on independent datums 

 depending on the triangulation in use. With 

 the completion of the first transcontinental 

 arc of triangulation the various detached 

 systems were connected and a coordinated 

 system based on a single geographic datum 

 was established for the whole country. Sta- 

 tion MEADES RANCH in central Kansas 

 was selected as the basis for this single 

 geodetic datum and in 1901 the adopted 

 datum was officially named the United States 

 Standard Datum. In 1913 Canada and Mexi- 

 co adopted this datum which was changed 

 in name only to the North American Datum. 



In 1927 a unified adjustment of all first 

 order triangulation in the country was begun 

 and, as a result, the North American Datum 

 of 1927 was adopted. This datum has been 

 extended into areas in Alaska where pre- 

 viously several independent datums were 

 used. Modem hydrographic surveys in wa- 

 ters bordering the continental United States 

 are now made on this geographic datum. 

 Surveys in the waters of the Hawaiian Is- 

 lands are based on the Old Hawaiian Datum. 



Many prior surveys list the geographic 

 position of a triangulation station positioned 

 within the surveyed area. It was frequently 

 inked in the lower margin, but the datum 

 is not always indicated. On later surveys 



