The measurement of a straight line perpendicularly downwards from a Downward elevation of a given point with reference to the logging reference point at surface:
TVD = TVDSS + Logging _ Reference _ Elevation
It is widely used in Subsurface Engineering.
See also
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Natural Science / Physics / Measuring / Positioning
[ Subsurface Engineering ] [ Altitude ]where Logging_Reference_Elevation is Excerpt Include