True vertical depth

True vertical depth[1] is the measurement of a straight line perpendicularly downwards from a horizontal plane.

In the petroleum industry, true vertical depth, abbreviated as TVD, is the measurement from the surface to the bottom of the borehole (or anywhere along its length) in a straight perpendicular line represented by line (a) in the image.

Line (b) is the actual borehole and its length would be considered the "measured depth"[2] in oil industry terminology.

If one were to imagine line (b) to be a piece of string, and further were to imagine it being pulled straight down, one would observe it to be longer than line (a).

This example oil well would be considered a directional well because it deviates from a straight vertical line.