Time-Dependent Transformations

Time-Dependent Transformations provide a means to predict and adjust datum transformations based on movements of the Earth's crust over time. Accounting for this movement and change over time can provide an additional level of accuracy for work that requires a high level of precision. Global Mapper currently supports a variety of time-dependent transformation methodologies. As geospatial positioning tools have developed and allowed for increased accuracy that accounts for the natural movement of the earth's surface, modern systems increasingly incorporate time.

Settings for Time-Dependent Transformations are available in the Coordinate Reference System tab of the Configure menu, when enabled in Geodetic Settings.

Global Mapper is able to perform several kinds of time-dependent transformations, as described below.

Horizontal Time-Dependent Positioning (HTDP)

Horizontal Time-Dependent Positioning (HTDP), is a horizontal datum transformation method created by the US National Oceanic and Atmospheric Administration's (NOAA) National Geodetic Survey (NGS) group. It is used to predict horizontal velocities and displace coordinates due to plate tectonics and seismic events. In Global Mapper, this logic is used to transform data across different reference frames and epochs. This capability is critical for maintaining geodetic integrity, as the tool was originally designed to align the North American Datum of 1983 (NAD83) with the International Terrestrial Reference Frame (ITRF) and WGS84.

HTDP quantifies the following changes to the earth's surface:

• Constant interseismic horizontal velocities (steady long term movement of tectonic plates)

• Coseismic motion (abrupt changes most often associated with earthquakes)

• Postseismic motion (longer term transient effects of earthquakes)

HTDP is comprised of two different methods. The first method utilizes velocities based on collected geodetic control stored in a set of grid files. Bilinear interpolation is used to calculate the relevant transformation applied to coordinates located within the bounds of the grid, roughly the United States. For data that falls outside of the defined bounds of these grid files, the second method is used. This second region type uses rigid tectonic plate models with global coverage.

In order to access HTDP transformations, the source reference epoch, and target reference epoch must be specified. Within Global Mapper, the source epoch needs to be assigned to the layer's source coordinate system which can be specified in the Layer Coordinate System tab of layer options. The target epoch must be specified in the Select Coordinate Reference System tab of Configuration.

Time-Dependent Helmert Transformations

This transformation type consists of a static 7 parameter Helmert, with defined X,Y,Z translations/rotations and scale at the reference epoch (t₀). Then each of those parameters has a defined rate of change at the reference epoch (t0), resulting in 7 additional parameters. In addition to those 14 parameters that describe starting conditions and expected change, the reference epoch is also written into the definition of the transformation as a "15th" parameter.

In order to access these transformations, a single matching date needs be entered in for both the source layer CRS, and the workspace CRS. The difference between the reference epoch and the user entered date is calculated, and multiplied by the rate of change for that specific parameter. This new calculated value is then added to the original static parameter, and the transformation is applied.

For example, consider the scenario of transforming between the ITRF2014 and ITRF96 datums using EPSG:9083. Below is an example of how the amount of X translation will be calculated assuming a user entered date of January 1st 2025. This same technique is applied to all of the static parameters, and then they are used for the transformation after accounting for the velocities.

In order to access time-dependent Helmert transformations, the user entered date specified for the layer, and the workspace need to be the same. The difference in time between this user entered date, and the reference epoch of the transformation is used to calculate the velocities applied to each of the transformation parameters.

Canadian Velocity Grid

The Canadian Velocity Grid is an NTv2 grid shift file with 3D velocity data that accounts for motion of the crust over time. This format supports transformation between different epochs of NAD83(CSRS) to account for the tectonic shift using the velocity field determined from active control stations.

Time-specific Coordinate Frame Rotation (CFR) and Time-Specific Position Frame Rotation

Time-Specific Coordinate Frame Rotation (CFR) and Time-Specific Position Vector Rotation (PVR) datum transformation methods are time-based transformations valid at the specific transformation reference epoch. In order for these 8 parameter transformations to become available when selecting a transformation, the source and target date must match the specific reference epoch for the transformation. These transformations contain the standard 7 parameters: X/Y/Z translations and rotations ,and a scale. The "8th parameter" is the transformation reference epoch.