105 



mil give the datum with the precision to which the gages can be read. 

 To estabUsh the datum for a project survey of a harbor or waterway 

 where no rehable datum is available, the observations should extend 

 over at least one period of 29 days. Such a comparison should es- 

 tablish the datum within a tenth of a foot if the base station is not too 

 remote. If less accuracy is needed, a comparison for a week may be 

 sufficient. 



207. In general a comparison with a suitable base station extending 

 over a year will give a determination of mean sea level within 0.05 

 foot of the long-term mean at the secondary station, and a comparison 

 extending over 4 years within 0.02 foot. (Special Publication 135, U. S. 

 Coast and Geodetic Survey.) The determination of mean sea level 

 at a station where a long record is not available is always improved by 

 ■comparing it with a primary station. 



FIXATION OF DATUM PLANES 



208. It has been seen that even the long-term means of the eleva- 

 tions of the various tidal datums change slightly as the records are 

 extended. Since changes in the datum on which successive surveys 

 are based tends to confusion and error, and makes a comparison be- 

 tween surveys a difficult and laborious process, an accepted elevation 

 of the adopted datum is established with respect to a stable bench 

 mark, or preferably a group of bench marks, as soon as this datum is 

 determined with sufficient precision. This datum is not thereafter 

 changed, unless new conditions make it grossly erroneous. 



209. Accuracy required. — So far as the usual purposes of navigation 

 and of harbor improvement are concerned, no high degree of precision 

 is required in the determination of a reference datum. The surface 

 of tidal waters is constantly changing in elevation, and may occasion- 

 ally be a foot or more below any of the datums used in the United 

 States. The squat of a vessel underway, and its pitch in rough water, 

 also render useless any refinements in the indicated depths. Hydro- 

 graphic charts therefore show the depths of inshore soundings to the 

 nearest foot, and offshore soundings in shoal areas to the nearest 

 quarter fathom (1.5 feet), and to the nearest fathom in deep water. 

 Channel depths are usually laid out to the nearest foot, although ordi- 

 narily the sounding from which estimates of dredging are prepared 

 are taken to the nearest tenth of a foot. The fixation of the reference 

 datum within a tenth of a foot or more of its true long-term mean is 

 therefore ordinarily sufficient. The stabilization of the datum is more 

 important than its inherent accuracy. 



210. Datums for dredging contracts. — In the administration of dredg- 

 ing by contract the definite fixation of the datum plane to cited shore 

 bench marks is essential. If the material removed is measured and 



