Using a Helmert Transformation

CONTEXT

You are working with two separate drawings:

• T1: a drawing in a local coordinate system, e.g. a site survey using a total station setup at (0, 0) or an architectural design plan.

• T2: a georeferenced plan, such as a cadastral map or a GPS-based terrain survey (e.g. using Lambert 72 coordinates).

Your goal is to accurately position T1 within T2 so both drawings align geographically and match perfectly.

Preparation: Define Control Points

To ensure accurate placement, you need at least 3 control points—preferably 4 or more for a more robust calculation.
These are points known in both the local (T1) and georeferenced (T2) systems.

Examples of control points: Building corners, parcel boundary markers, or recognizable infrastructure features.

TIPS

• Distribute the control points across the area.

• Avoid points that are too close or form a straight line.

• Always check the residuals: aim for a deviation less than 2 cm (St Dev < 0.02).

Make sure that in both T1 and T2, there are at least 3 points with the same unique point number. This number is the key Pythagoras uses to automatically match corresponding points during the transformation.

Check the Coordinate Reference System (CRS) of T2

Open T2 (the georeferenced drawing):

• Go to Settings > Document Settings > Coordinate Reference System.

• Select the correct CRS, e.g., Lambert 72 Belgium.

• Click OK.

⚠ Note: Each drawing can only have one active CRS.

Load Control Points & Perform the Transformation

1. Open T1 (the local drawing).

2. Select the control points (at least three).

3. Go to Calculation > Transformation > Helmert Transformation...

   1. Click Look up cooridinates.

      1. In the window 'Find XY Coordinates in', select your drawing T2.

      2. Click OK. 

   2. The corresponding coordinates will appear in columns X and Y.

   3. Click Calculate.

   4. The transformation parameters will be computed.

   5. Click OK.

RESULT

In T1, a new coordinate system 'Global' will appear in the control panel. The original 'Local' system remains available as well.

Copy Objects and Paste Using Global Coordinates

1. In T1, select everything via Edit > Find & Select > All Object Types (or use Ctrl+A).

2. Choose Edit > Copy > Pythagoras Clipboard (or Ctrl+C).

3. Switch to T2.

4. Choose Edit > Paste (or Ctrl+V).

5. Click OK on the message about duplicate (existing) point numbers.

RESULT

The content from the local drawing is now correctly positioned in the georeferenced environment.

⚠ Important Notes
Although it's technically possible to assign a CRS to the 'Global' system in T1, this is not necessary for this workflow. Assigning a CRS to 'Global' transforms the entire drawing permanently, which cannot be undone, and you’ll lose the original 'Local' system. Only do this if you're sure the project needs to be converted to one fixed coordinate system.