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 Production Engineering is usually using upward elevation notation while subsurface engineering is usually using downward elevation notation (see for example, True Vertical Depth (TVD)).
Natural Science / Physics / Measuring / Positioning
[ Elevation ] [ Northing] [ Easting ]