A chart showing the field of D-values (deviations of the actual altitudes along a constant- pressure surface from the standard atmosphere altitude of that surface) in terms of the three dimensions of space and one of time. It is a form of a four-dimensional display of pressure altitude. The space dimensions are represented by D-value contours, and the time dimension is provided by tau-value lines. 4D charts are used primarily for relatively long-distance preflight planning over ocean areas. See also S-values.