Horizontal Time Dependent Positioning (HTDP) provides a means to predict and adjust for data transformations related to movements of the Earth's crust over time.
HTDP coordinate reference systems in the Geographic Datasource can be identified by checking the horizontal datum they are based on. If the datum has an HTDP identifier, it is an HTDP system. The HTDP code will match up that datum to one of those defined in the NOAA, NGS HTDP software package (version 3) that can be found at the NGS website.
If you are starting the Geographic Calculator for the first time since installing the HTDP-enabled version, you will get the following informational message:
If you select No, the functionality of the Calculator will remain unchanged. If you select Yes, then when situations arise where HTDP is applicable, you will be able to use it. This setting can be changed within the Preferences Menu.
If HTDP is enabled, and you select an HTDP coordinate system for both the source and target for a given job, the Coordinate System display will be updated to include a date display:
If the Date field is checked, the specified date will be used to calculate the required HTDP datum transformation. If it is unchecked, no HTDP transformation will be applied, even if the other coordinate system does have an active date. Likewise if only one of the coordinate systems supports HTDP, the Date field will not be enabled. In this case you may receive the following warning if only one coordinate system is an HTDP system.
If both the source and target coordinate systems do support HTDP, and have active specified dates, then an HTDP transformation will be created for use by the current calculation. This datum transformation will not be added to the custom datasource and is not viewable in the Datasource dialogs. It is constructed based on the current transformation. The Datum Transformation field will be updated with text describing the transformation.
If the Workspace is saved at this point, the HTDP transformation will be restored when the application is restarted.