WGS84 / ITRS Epochs

WGS84 / ITRS Epochs / Continental Drift

GeoDLL supports ITRS-Epochs by using WGS84 coordinates from GPS measurements. Therefore in the list of the supported Coordinate Reference System group classifications "GPS Measurements - ITRS epochs" each for the European, the North American and the Austral Asian continent are available. WGS84 / ITRS GPS coordinates can be transformed with the respective parameter set of a required epoch with consideration of the continental drift to Local Coordinate Reference Systems.

Further information on this topic can be found in the publication "Are the Reference Systems WGS84 and ETRS89 really the same?" on the KilletSoft website https://www.killetsoft.de/t_1009_e.htm

Difference between WGS84 / ITRS and Local Reference Systems

The Reference System WGS84, used for GPS measurements, is based on the global International Terrestrial Reference System ITRS that is fixed on the earth's center of mass. Due to plate tectonics, the continents within the ITRS are slowly moving upon the surface of the earth in different directions.

Continent-wide Local Reference Systems, e.g. the ETRS89 or the NAD83 and all small-scale Local Reference Systems are fixed at points on the earth's surface. They quasi "move" on the global WGS84 / ITRS. The relationships between points within Local Reference Systems thereby remain stable.

Transformation between GPS coordinates and Local Coordinate Reference Systems

To maintain the high accuracy of GPS long-term measurements, for the transformation of GPS coordinates from the WGS84 / ITRS Reference System to Local Reference Systems exact transformation parameters are required. For this, precise parameter sets are calculated every couple of years and made public as ITRS realizations. The WGS84 / ITRS GPS coordinates are converted to Local Reference Systems of the continents, e.g. ETRS89 for Europe and NAD84 for North America, using the parameter set of their epoch. For an exact calculation, one therefore needs to know the epoch, i.e. the year in which the GPS data were recorded or measured. These epochs, also called annual realizations, are issued using the year in the form of ITRS89 for the year 1989, successively up to the year ITRSxx. Where xx stands for the year of the last GeoDLL update, which is also referred to as the "license year".

Due to the occasional new ITRS realizations, it is not possible to enter the ITRS epochs for all future years in the Geodetic Development Tool in advance. Only one or two ITRS epochs that lie in the future following the license year are supported. An automatic adjustment, e.g. by the time variable of the operating system, cannot be used either because this can be changed by the user. As a result, older GeoDLL versions cannot be used to perform highly accurate conversions with current ITRS epochs. However, the all-clear can be given for most calculations. Due to continental drift, the accuracy only changes by a few centimeters each year. For the European plate, this is approximately 2.5 centimeters per year. If this accuracy is not sufficient, an economical priced update to the current version of the GeoDLL can be purchased from KilletSoft, in which the ITRS parameters for the current epochs are already implemented.

Support of GPS Epochs

GeoDLL and the coordinate conversion program TRANSDAT developed by KilletSoft, provide the module "GPS Measurements - ITRS epochs" that can carry out very precise Coordinate Transformations from current GPS measurements to Local Reference Systems using the parameter set ITRSyy for the license year. In addition, the module enables the transformation of GPS coordinates to Local Reference Systems of any older epochs and of one or two future epochs. The inverse transformation from Local Reference Systems to any WGS84 / ITRS epochs is also possible. To align GPS coordinates, it is also possible to transform data from one WGS84 / ITRS epoch to another.

The available ITRS parameter sets will be dynamically continued in the coming years. By using the ITRS parameter set of the license year (see above) for all conversions between WGS84 and Local Reference Systems the highest possible accuracy will be reached. Current WGS84 coordinates can be converted without loss of accuracy due to continental drift to other Reference Systems.

Examples

You have Geographic Coordinates from a GPS measurement campaign that took place 1998 in Europe. You want to convert these coordinates to the Coordinate Reference System UTM / ETRS89. You must make the following settings:

Source Coordinate System: Geographic coordinates (Greenwich) [deg] GeoDLL 6

Source Reference System: ITRS98 (EU GPS measurements epoch 1998), WGS84 GeoDLL 234

Target Coordinate System: UTM coordinates (northern hemisphere) GeoDLL 3

Target Reference System: ETRS89 (EU), geocentric, GRS80 GeoDLL 4

You have Geographic Coordinates from a GPS measurement campaign that took place in 2001 in the USA. You want to convert these coordinates to the Coordinate Reference System UTM / NAD83. You must make the following settings:

Source Coordinate System: Geographic coordinates (Greenwich) [deg] GeoDLL 6

Source Reference System: ITRS01 (US+CA GPS measurements epoch 2001) GeoDLL 386

Target Coordinate System: UTM coordinates (northern hemisphere) GeoDLL 3

Target Reference System: NAD83(NSRS 2011) (US), geocentric, GRS80 GeoDLL 447

You have a file with border coordinates in the Coordinate Reference System MGA / GDA94 from Australia and would like to convert these into the new highly accurate GDA2020 for controlling a GPS driven farm machine. You must make the following settings:

Source Coordinate System: MGA - Map Grid of Australia (AU) Transv. Merc. GeoDLL 587

Source Reference System: GDA94 (AU), geocentric, GRS80 GeoDLL 83

Target Coordinate System: Geographic coordinates (Greenwich) [deg] GeoDLL 6

Target Reference System: GDA2020 (AU fixed for GPS epoch 2020), GRS80 GeoDLL 640