True vertical depth
True vertical depth 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 in a straight perpendicular line represented by line in the image.
Line is the actual borehole and its length would be considered the "measured depth" in oil industry terminology. The TVD is always equal to or less than the measured depth. If you imagine line were a piece of string and pull it straight down, you would see that it would be longer than line. This example oilwell would be considered a directional well because it deviates from a straight vertical line.
