130 U. S. COAST AND GEODETIC SURVEY 
hydrographic charts of that port. This consideration limits the practical datums for 
such purposes to some low-water datum like mean low water, lower low water, or spring 
low water. 
The Indian tide plane, or the harmonic tide plane, as it is sometimes called, has 
been used for a number of ports in India. It is defined as the datum plane that lies 
below mean sea level a distance given by adding the amplitudes of the principal lunar 
semidiurnal, principal solar semidiurnal, the principal lunisolar diurnal, and the principal 
lunar diurnal components. In the accepted notation it may be written as follows: 
MSL—(M,+8,+K,+0,). The Indian tide plane is sometimes called also the plane 
of Indian spring low water. From its definition in terms of the harmonic constants it 
obviously corresponds approximately to tropic spring lower low water. 
