Internet searches often imply that the Reference System WGS84, which is used with GPS measurements, and the European Reference System ETRS89 are virtually identical and that Coordinate Transformation between the two systems is unnecessary. In fact, the positions calculated by these two Reference Systems currently deviate by more than half a meter. For the purposes of navigation or geodetic infonomics, this does not present a problem as here the need for precision is negligible, and thus a conversion between the two systems is superfluous. However, for high-precision surveying work within the framework of geodesic spatial referencing and for data with legal effects, like the Land Survey Register, this difference must be accounted for.
The target audience for this scientific article is users with geodesic expertise, developers of geodesic software, GIS consultants and engineering offices, as well as administrative GIS officers who deal with GPS surveying and/or different Coordinate Reference Systems (CRS).
Coordinate Reference System
A CRS contains two different elements - the Coordinate System and the Reference System (datum).
The Coordinate System describes how the coordinates of the CRS are expressed by calculation rules and projection equations. Examples: Ellipsoidal coordinates, Cartesian coordinates, UTM, Gauss Kruger.
The Reference System defines how the CRS is related to the earth. Examples: ED50, DHDN, ETRS89.
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 Eurasian continent within the ITRS is slowly moving upon the surface of the earth in a north-easterly direction.
Img. 1: Difference between ITRS and ETRS89
Source: Prof. Dr.-Ing. Wolfgang Augath;
Beiträge des Vermessungswesens zur Ortung
und Navigation im Wandel; Dresden Continental Drift
Since ETRS89 was realized until today (2012), a difference of 50 to 60 centimeters has accrued between WGS84/ITRS and ETRS89 which continues to grow dynamically at a rate of approximately 2.5 centimeters annually. The difference varies depending upon the location of a given coordinate on the Eurasian plate, because the continent slowly but steadily rotates around the Earth center of mass. The vectors representing displacements and the angles representing rotations are graphically depicted in Img. 1.
Converting GPS coordinates to ETRS89
For direct measurements the conversion to ETRS89 is usually not necessary, as it in terms of the systemic high inaccuracy of several meters would not lead to a significant improvement. Here, the equating of WGS84 and ETRS89 can be tolerated. However, to maintain the high accuracy of DGPS and SAPOS, for the transformation of GPS coordinates from the WGS84/ITRS Reference System to ETRS89 exact transformation parameters are required. To this end, precise parameter sets are calculated and regularly made public as ITRS annual realizations or WGS84 epochs. The WGS84/ITRS GPS coordinates are converted to ETRS89 with their given epoch's parameter set. For an exact calculation, it is therefore necessary to know the epoch, that is, the year the GPS data was implemented. These realizations are issued using the year in the form of ITRS89 for the year 1989, successively up to ITRS12 for the year 2012 and annually ongoing.
TRANSDAT supports GPS epochs
KilletSoft's TRANSDAT coordinate conversion program provides a module "GPS Measurements - ITRS epochs" that can carry out very precise Coordinate Transformations from current GPS measurements to the Reference System ETRS89 using the parameter set ITRSyy for the current year. In addition, the module enables the transformation of GPS coordinates to ETRS89 of any older epochs since 1989 and of future epochs. The inverse transformation of ETRS89 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. For all this special transformations various geographic notations and UTM coordinates are used as Coordinate Systems.
A sample calculation with TRANSDAT
Img. 2: Sample calculation with TRANSDAT For example, here will be shown how to convert highly accurate GPS coordinates from an earlier epoch to the Europe-wide CRS UTM/ETRS89 specified by INSPIRE with consideration of the continental drift.
Precise Coordinate Transformations in own applications
To incorporate the geodesic calculations used by the TRANSDAT program into programs of your own, the GeoDLL dynamic link library is at your disposal. All of the geodesic calculations included here are based on strict formulas that we use to attain high-precision results. GeoDLL also includes the ITRS parameter set releases and the essential high-precision transformation routines. To ensure that the geodesic functions can be implemented on other platforms, the GeoDLL C++ source code is also available.
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