(a) is an imaginary line representing the True vertical depth, while line (b) is the borehole itself, and its length is called the measured depth. A borehole is the generalised term for any narrow shaft drilled in the ground either vertically or horizontally

Measured depth[1], as it is called in the oilfield, is the length of the borehole, Line (b) in the image, and is measured in feet. The petroleum industry includes the global processes of exploration, extraction, refining, transporting (often by Oil tankers and pipelines A borehole is the generalised term for any narrow shaft drilled in the ground either vertically or horizontally The borehole in this image is direction but many oilwells are still just conventional vertical wells (straight holes). West Texas PumpjackJPG|thumb|right|300px|This Pumpjack located south of Midland TX is a common sight in West Texas.

Measured depth is normally referred to as just depth except when the oilwell is directional (as is the case in the image) or horizontal. West Texas PumpjackJPG|thumb|right|300px|This Pumpjack located south of Midland TX is a common sight in West Texas. A Directional well, as it is known in the oilfield, is any Borehole that deviates from a vertically straight line In these wells, certain engineering calculations depend upon the true vertical depth, Line (a), to determine certain fluid mechanics. True vertical depth is the measurement of a straight line Perpendicularly downwards from a Horizontal plane. Fluid mechanics is the study of how Fluids move and the Forces on them

The True vertical depth is always equal to or less than (≤) the measured depth. True vertical depth is the measurement of a straight line Perpendicularly downwards from a Horizontal plane. If you imagine Line (b) were a piece of string and pull it straight down, you would see that it would be longer than Line (a).