Depth Choice Qualifier depthChoiceType POSC 1.0 MD Measured Depth. The depth values are given in measured depths, measured from a specified datum, as distance along a borehole. The distances are positive with increasing distance along the borehole. TVD True Vertical Depth. This is the vertical, straight line depth from the surface datum reference to the subsurface point of interest. TVD is given as positive downward. Subsea TVD This is the vertical, straight line depth from the surface datum reference to the subsurface point of interest. This differs from TVD in that the values are given as positive upward. Hence most subsurface Subsea TVD values are negative. By default, the values are measured with respect to mean seal level. However, another datum may be specified, if the model allows it.