PREFACE TO SECOND EDITION 
In this revision of ‘Tidal Datum Planes,” the first edition of which was issued in 
1927, changes have been made to conform with the most recent practice of the Coast 
and Geodetic Survey. The section on lunitidal intervals has been eliminated since this 
matter is only of secondary interest in datum planes and is adequately covered in other 
Survey publications. The section on mean sea level has been extended to include the 
question of changing sea level, and consideration is given to datum planes of tides 
predominantly diurnal. 
PREFACE TO FIRST EDITION 
Of datum planes that may be used as planes of reference for elevations, those 
based on the rise and fall of the tide have the advantages of simplicity of definition, 
accuracy of determination, and certainty of recovery, even though all bench-mark 
connection be lost. Tidal datum planes are, therefore, the basic planes of reference 
used in the hydrographic and geodetic work of the Coast and Geodetic Survey. 
In the present publication two objects have been kept in mind. It is aimed to 
provide a working manual for the determination of the more important tidal datum 
planes and at the same time to provide a sufficient discussion of the principles in- 
volved and accuracy attainable. Since no such discussion is elsewhere available, this 
phase of the subject has been treated in detail. 
A considerable body of observational material formed the basis of the investiga- 
tions here undertaken, long-continued observations being especially important. Of 
these latter, however, those at hand were limited almost without exception to the 
observations made by this bureau on the coasts of the United States. It is for this 
reason, and for the further reason that the publication is to serve as a manual in the 
Coast and Geodetic Survey, that the examples chosen are from observations on the 
coasts of the United States. 
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