The Helmert transformation is a transformation method within a three-dimensional space. It is frequently used in geodesy to produce distortion-free transformations from one datum to another. The Helmert transformation is also called a seven-parameter transformation and is a similarity transformation.
It is not always necessary to use the seven parameter transformation, sometimes it is sufficient to use the five parameter transformation, composed of three translations, one rotation and one change of scale. A special case is the two-dimensional Helmert transformation. Here, only four parameters are needed. These can be determined from two known points; if more points are available then checks can be made.
Restrictions
The Helmert transformation only uses one scale factor, so it is not suitable for:
The manipulation of measured drawings and photographs
The comparison of paper deformations while scanning old plans and maps.
The Helmert transformation is used, among other things, in geodesy to transform the coordinates of the point from one coordinate system into another. Using it, it becomes possible to convert regional surveying points into the WGS84 locations used by GPS. In the process, the Gauss–Krüger coordinate, and, plus the height,, are converted into 3D values in steps:
Calculation of the ellipsoidal latitude, longitude and height
Because of this, terrestrially measured positions can be compared with GPS data; these can then be brought into the surveying as new points – transformed in the opposite order.
The third step consists of the application of a rotation matrix, multiplication with the scale factor and the addition of the three translations,,,. The coordinates of a reference system B are derived from reference system A by the following formula: or for each single parameter of the coordinate: For the reverse transformation, each element is multiplied by −1. The seven parameters are determined for each region with three or more "identical points" of both systems. To bring them into agreement, the small inconsistencies are adjusted using the method of least squares – that is, eliminated in a statistically plausible manner.
Standard parameters
Note that the rotation angles given in the table are in seconds and must be converted to radians before use in the calculation.
These are standard parameter sets for the 7-parameter transformation between two datums. For a transformation in the opposite direction, inverse transformation parameters should be calculated or inverse transformation should be applied. The translations,, are sometimes described as,,, or,,. The rotations rx, ry, and rz are sometimes also described as, and. In the United Kingdom the prime interest is the transformation between the OSGB36 datum used by the Ordnance survey for Grid References on its Landranger and Explorer maps to the WGS84 implementation used by GPS technology. The Gauss–Krüger coordinate system used in Germany normally refers to the Bessel ellipsoid. A further datum of interest was ED50 based on the Hayford ellipsoid. ED50 was part of the fundamentals of the NATO coordinates up to the 1980s, and many national coordinate systems of Gauss–Krüger are defined by ED50. The earth does not have a perfect ellipsoidal shape, but is described as a geoid. Instead, the geoid of the earth is described by many ellipsoids. Depending upon the actual location, the "locally best aligned ellipsoid" has been used for surveying and mapping purposes. The standard parameter set gives an accuracy of about for an OSGB36/WGS84 transformation. This is not precise enough for surveying, and the Ordnance Survey supplements these results by using a lookup table of further translations in order to reach accuracy.
Calculating the parameters
If the transformation parameters are unknown, they can be calculated with reference points have to be determined, at least two points and one coordinate of a third point must be known. This gives a system with seven equations and seven unknowns, which can be solved. In practice, it is best to use more points. Through this correspondence, more accuracy is obtained, and a statistical assessment of the results becomes possible. In this case, the calculation is adjusted with the Gaussian least squares method. A numerical value for the accuracy of the transformation parameters is obtained by calculating the values at the reference points, and weighting the results relative to the centroid of the points. While the method is mathematically rigorous, it is entirely dependent on the accuracy of the parameters that are used. In practice, these parameters are computed from the inclusion of at least three known points in the networks. However the accuracy of these will affect the following transformation parameters, as these points will contain observation errors. Therefore, a "real-world" transformation will only be a best estimate and should contain a statistical measure of its quality.
