68 U. S. COAST AND GEODETIC SURVEY 
Reference to Tide Staff and Bench Marks 
From the tide observations at any point the plane of mean sea level is determined 
as corresponding to a certain height on the fixed tide staff used in the tide observations 
at that point. In other words, mean sea level at that point may be said to be so many 
feet and hundredths above the zero of a given tide staff. And if it were a simple matter 
to maintain that tide staff for many years without change in elevation, it would serve 
very well for preserving the determination of the plane of mean sea level. 
But unfortunately it is only rarely that a tide staff can be maintaimed without 
change for a number of years. Deterioration of the material used, changes in wharves 
and piling, and accidents of one kind or another make it necessary to replace a tide 
staff at intervals more or less frequent. To make certain that the new tide staff will 
be replaced at the same elevation as the preceding one, so as to make succeeding obser- 
vations comparable with those preceding, it is necessary to refer the zero of the tide 
staff to bench marks. 
As soon as the elevation of the tide staff with reference to one or more bench 
marks is known it becomes possible to refer the determination of the plane of mean 
sea level to these bench marks. These bench marks, established in rock or concrete, 
or on some substantial structure, thus preserve the determination of mean sea level 
much better than would the tide staff; and generally this plane at any point is given 
as so many feet and hundredths below one or more bench marks. 
