True vertical depth (TVD) is usually expressed as the vertical distance below RKB. Departure is the distance between two survey points as projected onto the horizontal plane. The EOB is defined in terms of its location in space as expressed by coordinates and TVD. The EOB specification also contains another important requirement, which is the angle and direction of the well at that point. The correct angle and direction are critical in allowing the next target to be achieved; also, it may be necessary to penetrate the pay-zone at some optimum angle for production purposes.