The **Helmert transformation** (named after Friedrich Robert Helmert, 1843–1917; also called a **seven-parameter 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 using:

where

*X*_{T}is the transformed vector*X*is the initial vector

The parameters are:

- — translation vector. Contains the three translations along the coordinate axes
- — scale factor, which is unitless, and as it is usually expressed in ppm, it must be divided by 1,000,000.
- — rotation matrix. Consists of three axes (small rotations around the coordinate axes), . The rotation matrix is an orthogonal matrix. The rotation is given in radians.

Thus, the Helmert transformation is a similarity transformation.

Read more about Helmert Transformation: Calculating The Parameters, Two-dimensional Case, Application, Standard Parameters, Restrictions

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