Compute a 2D Helmert from common points

This is the common-point transformation, not a named EPSG operation. Open Calculations > Transformations.

Two tabs: Parameter Calculation finds the parameters from points known in both systems. Transform Points applies those parameters to other points.

Helmert 2D

Helmert 2D is a similarity: one scale, one rotation, and a shift in X and Y. It keeps angles. The minimum is 2 common points. Three or more give a residual.

1. Choose Helmert 2D in the model list.
2. Enter or import the source points in the left grid and the target points in the right grid. The same point occupies the same row in both.
3. Click Calculate.
4. Read the residuals. Switch a point off if it is an outlier, and calculate again.
5. Open Transform Points, import the points that only exist in the source system, and click Transform.
6. Export the result.

The other models in the same list are Affine 2D (6 parameters, minimum 3 non-collinear points), a 2nd-order or 3rd-order polynomial, and the 3D Helmert and affine models. Use Helmert 2D when the two systems differ by a shift, a rotation and a single scale — a local survey against a control grid, for example. Use the affine model when the scale in X and the scale in Y are not the same.