Time-Dependent Transformations provide a means to predict and adjust for horizontal datum transformations related to movements of the Earth's crust over time. Horizontal Time Dependent Positioning (HTDP), the Time-Dependent Helmert transformation, and Canadian Velocity Grid are options for this type of transformation available in Geographic Calculator.

Canadian Velocity Grid

The Canadian Velocity Grid accounts for shifts in tectonic plates between epochs. For more information see Canadian Velocity Grid

Time-Dependent Euler Pole Transformations

1This function has been added as part of the NOAA National Geodetic Survey NSRS and Canadian Geodetic Survey CSRS modernization (e.g. NATRF2022); see NGS Beta Products for more information.

Based on Euler's Fixed Point Theorem, Euler Pole Parameter (EPP) transformations model plate movement as a rigid rotation around a specific fixed axis (the Euler Pole) passing through the center of the Earth. This method accounts for global crust movement by applying angular rotation rates over the elapsed time between a reference epoch and an observation date. National Geodetic Survey modernization includes EPP2022 (Euler Pole Parameters 2022) for converting coordinates between ITRF and the new NATRF2022, PATRF2022, CATRF2022, and MARF2022 reference frames. To enable this transformation, be sure to specify the survey epoch (date of observation) on both the Source and Target date fields (i.e., the same date must be entered in both Source and Target fields).

HTDP

HTDP coordinate reference systems in the Geodetic 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.

Time-Specific Coordinate Frame Rotation (CFR) and Time-Specific Position Vector Rotation (PVR)

The Eight Parameter EPSG transformations can be CFR (coordinate frame roatation) or PVR (position vector rotation) methods. As time-fixed transforms these transformations are only valid a a given time. For more information see Time-Specific CFR and PVR transformations.

Time Dependent Helmert Transformations

Time-Dependent Helmert Transformations transform between two known coordinate systems. Some exist in the datasource by default. They can also be defined in the datasource, see Datum Transformation Definitions for more information.

GGXF Dynamic Geoid Model (Time-Dependent Geoid)

1This function has been added as part of the NOAA National Geodetic Survey NSRS and Canadian Geodetic Survey CSRS modernization (e.g. NATRF2022); see NGS Beta Products for more information.

A Dynamic Geoid Model (such as the dynamic version of GEOID2022 for NAPGD2022) includes not only the baseline geoid undulation but also the linear time rate of change in undulation (meters/year) caused by Earth’s mass redistribution. To use the Dynamic Geoid, ensure that time-dependent transformations are enabled in Options > Preferences, and specify the survey epoch (date of observation) on both the Source and Target date fields (i.e., the same date must be entered in both Source and Target fields to enable the transformation).

New Zealand Deformation Model

The New Zealand Deformation Model, maintained by Land Information New Zealand (LINZ) as a core component of the NZGD2000 reference frame, is a time-dependent spatial system designed to correct for ongoing crustal movements along the active Pacific-Australian tectonic plate boundary. To enable this transformation, be sure to specify the survey epoch (date of observation) on both the Source and Target date fields (i.e., the same date must be entered in both Source and Target fields).

Selecting Time Dependent Transformations

By default, date fields will be enabled for all coordinate system definitions. This setting can be changed within the Preferences Menu. When the date field is enabled, click on it to select a date:

If the time-dependent transformations are enabled in Preferences, and the Date fields are set, any valid time-dependant datum transformations will be included in possible results for your Coordinate Transformation.

In the case of HTDP selection: 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.

In the Interactive Conversion Job, the HTDP Velocity values may be viewed by right-clicking on the output values in the Coordinate Point Definition field for the transformation output and selecting the 'HTDP Velocity' options.

In the Point Database Conversion Job, be sure that the "Show Time Dependent Velocities" option is enabled in Options>Preferences>Point Database. The boxes will appear in the Point Database Conversion menu when a Time Dependent Transformation is selected.

Selecting this option will bring up a window (below) displaying the East, North and Up velocity values in millimeters per year.

"North" velocity is measured as true north for geodetic, or grid north for projected.

"Up" velocity is measured as change in elevation from the location you started along an imaginary line that is perpendicular to the ellipsoid.