The Helmert transformation is used if you want to linearly scale objects or an image based on reference points. The result is a horizontal transformation (X and Y). A height transformation (Z) is also possible. Usually the horizontal transformation suffices (rotation and translation).
- Open a new document.
- Go to the menu: File - Import - Image - Link to file… .
- It is possible to trim the image. For example, choose the tool: ‘Cut off outside images’, in the drawing tools ‘Edit’. Click to define first corner of clipping rectangle. Drag to define the second corner.
- Draw at least 3 points on the image corresponding to points in your reference drawing.
- In both documents, rename these points so they have the same point number. For example: 1001, 1002, 1003.
- Select the 3 points in the drawing with the image to transform, and go to the menu: Calculation - Transformation - Helmert transformation… .
- You refer to the points of the reference drawing which is also opened in Pythagoras and click: 'Look up coordinates'.
- Check “Calculate scale” and uncheck “Transform Z”, and press: Calculate.
- In the columns f(X) and f(Y) the differences between base and reference are calculated. This results in a general translation in X and Y, a rotation and thus a standard deviation. If the value for standard deviation is too high, the set of base points does not correspond to the set of reference points and you need to evaluate if the constellation of the two sets matches.
- If the standard deviation is acceptable press OK to finalize the transformation.
- A 'Global CS' is created.
- Press the icon in the top menu, choose ‘Global CS' in the dialog box and choose the correct coordinate system.