106 



paid for in place, as computed from soundings taken before and after 

 dredging, the systematic error introduced by the variation of even a 

 tenth of a foot between the datums of the two surveys might, in a large 

 contract, result in overpayments or under payments amounting to 

 thousands of dollars. Even when the material is measured in the 

 scows into which it is loaded, the difference in the deductions made for 

 material removed below the plane of tolerance for payment might 

 amount to a considerable sum. The reference bench marks cited in 

 the specifications should be verified before the specifications are issued. 



211. Unless the tidal range differs materially in different parts of a 

 harbor or waterway, the datum for harbor improvement is taken as 

 that at one selected tidal station. The datum at all other points is 

 then taken as at the same elevation, this elevation being determined 

 either by lines or levels on shore, or by water levels established by the 

 half tide level corrected by comparison with tile base station. Wlien, 

 however, the tidal range, and consequently the elevation of the adopted 

 low water datum below mean sea level, differs materially along the 

 waterway, a succession of reference planes should be used, each appli- 

 cable to definitely defined sections or areas, and all correlated to a 

 common datum, preferably mean sea level. Thus on successive sec- 

 tions of the East River, N. Y., some 8 datum planes of mean low water 

 are used, varying in elevation from 1.96 to 3.51 feet below mean sea 

 level. 



TIDAL OBSERVATIONS 



212. Staf and automatic gages. — Tidal observations to establish 

 tidal datums, to provide the data for the harmonic analysis of the 

 tide, or to show the varying height of the water with respect to the 

 datum during surveys and dredging operations, are taken on staff 

 or on autorriatic gages. The staf gage is a graduated board, usually 

 set vertically, on which the height of tide is read by an observer. It 

 is ordinarily graduated in feet and tenths, with bold markings so 

 that it can be read at a distance. An automatic gage is a device by 

 which the elevation of a float is recorded, on a reduced scale, on a 

 moving paper driven by clockwork. The float is enclosed in a box 

 or pipe, with a restricted entrance near the bottom, to dampen the 

 fluctuations due to wind waves. In cold climates this box is filled 

 with kerosene to prevent freezing. A staff gage is always installed 

 with an automatic gage, the zero of the staff establishing by direct 

 comparison the zero of the record. Two types of automatic gages 

 have been developed by the United States Coast and Geodetic Survey, 

 one a more elaborate instrument for permanent stations at which 

 long-term records are maintained, and the other a portable type for 

 the temporary occupation of a station. 



