The distance measurement along upward or downward perpendicular to the sea level.
The reference point from which the distance is measured depends on context ( see Logging Reference Point ).
The Surface Engineering is usually using upward elevation notation which is also called Altitude.
The Subsurface Engineering is usually using downward elevation notation which is also called True Vertical Depth (TVD).
See also
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Natural Science / Physics / Measuring / Positioning
[ Elevation ] [ Northing] [ Easting ]
[ Altitude ] [ True Vertical Depth (TVD) ]