The lateral line is on the side of the shark

Hydraulic grade line is sum of Datum + Pressure Head

Energy grade line is sum of Datum + Pressure Head + Velocity Head

The lateral line belongs to the lateral line system. This helps to stream line fish in water and to keep them afloat.

Land-surface datum is a datum plane that is approximately at land surface at each well. If known, the elevation of the land-surface datum above sea level is given...

Datum: any level surface, line, or point used as a reference in measuring elevations.

Datum is an imaginary vertical plane from which all horizontal measurements are taken with the aircraft in a level flight attitude.