RotorFit

RotorFit is a tool for surveyors and engineers. From the coordinates of a few points measured on a circular-section object, the app finds the center, radius and diameter of the circle, along with the best-fit plane the circle lies in.
The app works for any circular-section structure. It can be a tank, a gas holder, a chimney, a water tower, a mine shaft, a tunnel lining ring, a flange or a pipeline.
The survey points do not have to share the same elevation. The only requirement is that they lie as close as possible to a single plane. That plane may be tilted: for example, the cutterhead (rotor) of a tunnel boring machine (TBM) is often mounted at an angle to the horizontal. RotorFit handles this correctly because it builds the plane from the points themselves rather than tying it to the horizon.
Features:

Circle center and radius from the measured points.
Best-fit plane and its normal.
RMSE (root-mean-square error) — a numeric measure of fit quality, plus the residual for each point to help spot gross errors.
A second circle offset along the plane normal. The offset can be positive, zero or negative, which gives the adjusted center of the second circle. This center is convenient for checking installation accuracy (for example, the design position of a ring or flange relative to the measured plane).
Division of the second circle into a chosen number of equal parts — ready-made points for setting out and stake-out.

Data formats and export:

Coordinates can be pasted from the clipboard, entered manually, or loaded from a TXT file.
Different line formats are supported: with or without a point ID; separators can be a comma, a semicolon or a space.
Results and the second-circle points can be copied or saved to TXT — in a format convenient for AutoCAD and other software.

3D view:

The cloud of measured points and both circles shown in 3D.
Drag to rotate, pinch to zoom.
Tap a point to open a panel with its ID and coordinates, which you can copy.
At the top, the tilt angles of the normal to two planes are shown — in degrees and in percent (slope, where 100% corresponds to a 45 angle).

How it works:
The app builds the best-fit plane using the least-squares method — the plane closest to the input points, where the sum of the squared deviations of the points from the plane is at a minimum. The points are then projected onto this plane, and a circle is fitted to them using the robust Taubin method. All calculations run on the device, with no internet connection required.

Works fully offline. No ads.