A true vertical depth is a depth vertically below a reference point to a point within a well. It is the depth perpendicular to the Earth’s surface and is often referenced to sea level. Wells are rarely drilled vertically but are deviated to a greater or lesser amount to some degree. A depth indicated by pipe or wireline is actually a length that is not directly below the surface location. It is calculated from knowing the continuous orientation of the borehole.
See also total depth and true stratigraphical thickness.