Hello,
I would like to share my new multipage app:
https://2d-helmert-transformation.streamlit.app/
A 2D Helmert transformation is used to transform coordinates from one rectangular source coordinate system to another target coordinate system. It is also known as 4 parameter similarity transformation, it is the most common planar transformation method in surveying.
The user can upload files with source and target coordinates, but if no files are uploaded, example coordinates are used, so you can also try the app without uploading your own files.
At least 2 points with known coordinates in both source and target system must be known to determine the transformation parameters. If more than 2 common points are given, a standard deviation and residuals of the common points are calculated. The user can unselect or select common points and the transformation parameters are re-calculated after every change in the selection. The streamlit component aggrid was used for this.
If you have any comments or suggestions, please feel free to share them. 
