TIDAL DATUM PLANES 67 
the 1924-1942 value of mean sea level on staff is 6.31, the yearly sea levels for 1933 
and 1941 being, respectively, 6.15 and 6.58 feet. The corrections for those years 
are therefore +0.16 and —0.27, which give mean sea level values at Los Angeles of 
6.43 feet for 1933 and 6.46 feet for 1941, differing from the 1924-1942 value at Los 
Angeles by 0.05 and 0.02 foot, respectively. 
Using San Diego as the primary station and deriving mean sea level for each of 
the 25 years of observations at Los Angeles, it is found that the average difference 
from the primary determination is 0.03 foot, while the greatest difference is 0.08 foot. 
In general it may be taken that mean sea level determined from a year of observa- 
tions, when compared with simultaneous observations at a suitable primary tide 
station will give a value correct to within 0.05 foot. 
Three Years.—From the nature of the case it is clear that the longer the series 
the more precise is the determination of mean sea level. Where a continuous series 
of observations is being made, a preliminary determination is derived from a year of 
observations. This value is then maintained until a 3-year series is available, when 
a more precise determination is made if required. 
The accuracy with which a 3-year series gives mean sea level, after correction by 
comparison, may be exemplified by taking Boston and Baltimore as in the example 
for 1 year. Forming 3-year running means of the 27 years of observations at Boston 
and correcting these to mean values by comparison with similar 3-year means at 
Baltimore, the average difference between these mean sea level values and the primary 
determination is 0.03 foot, while the greatest difference for any one 3-year group 
is 0.08 foot. These differences compare with the corresponding differences derived 
from 1 year of observations of these two stations of 0.04 foot and 0.13 foot. 
Nine Years.—From the Boston and Baltimore observations for 1922-1948, 19 
running 9-year means may be derived. Correcting these 9-year means to a 19-year 
value and comparing with a primary determination it is found that the average dif- 
ference is 0.016 foot while the greatest difference for any one 9-year group is 0.03 foot. 
Primary Tide Stations 
The possibility of determining the plane of mean sea level from short series of 
observations is thus seea to depend on the existence of tide stations at which long 
series of observations are being made. Such tide stations are designated as primary 
tide stations. At the present time the Coast and Geodetic Survey is operating 30 
such stations on the Atlantic coast, 8 on the Gulf coast, 15 on the Pacific coast, and 
8 in Alaska. . 
The primary tide stations serve a number of purposes. They furnish primary 
determinations of mean sea level at these stations, which are then used as the starting 
and ‘‘tie-in’”’ points of the precise level net which is being spread over the country. 
The data also permit the precise determination of other tidal datum planes at these 
stations and make possible the correction to mean values of the results of short series 
of tide observations in the vicinity. ‘The records furthermore furnish data on the 
heights of the tide at any particular time, on the slow changes taking place in the 
relative elevations of land and sea and also the basic data for the study and advance- 
ment of the subject of tides. 
