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:
— 三个坐标轴的平移参数（已知量）
— 转换的尺度变化参数
— 含有三个坐标轴旋转参数的旋转矩阵，为一正交矩阵。