Horizontal Time-Dependent Positioning (HTDP), is an horizontal datum transformation method that was created by the US National Oceanic and Atmospheric Administration's (NOAA) National Geodetic Survey (NGS) group. It a time based model that provides a mechanism for converting between two relative models of the earth by taking into account several factors. These include crustal velocities from earth plate movement, displacements caused by natural or man-made phenomena, and adjustments created for local reference frames. It was originally designed as a method for aligning the NAD83 reference frame to the ITRF and WGS84 frames, but it can also be used to model changes independent of NAD83. In order to take advantage of this transformation, the user will have a two dimensional point value with its associated collection date. They will need to provide a required output reference frame (Geodetic Coordinate system with attached HTDP datum) and a realization time.
See also Time Dependant Transformations