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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 which is also called Altitude.
The Subsurface Engineering while subsurface engineering is usually using downward elevation notation (see for example, 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) ]
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