The transform is the heart of SiteLens: it computes the relationship between your building grid and real-world projected coordinates.
What it solves
SiteLens fits a 4-parameter Helmert (similarity) transform:
- Translation — a shift in easting and northing.
- Rotation — how the grid is rotated relative to the projection.
- Scale — a single uniform scale factor.
This is the right model for tying a rigid building grid to ground control: it moves, rotates, and scales the grid as a whole without distorting its shape.
Exact vs. least-squares
- With two control points, the four parameters are solved exactly.
- With three or more, the system is over-determined and SiteLens fits the parameters by least squares — the best compromise across all points.
Reading the residuals
After solving, SiteLens shows a residual at each control point: how far the fitted grid lands from the published coordinate, in easting and northing plus a magnitude. It also reports the overall RMS error.
Use these to judge the tie:
- Small, evenly distributed residuals mean a clean fit.
- One large residual usually means a bad control point or a transcription error — check that point's coordinates.
- A high RMS across the board can indicate a wrong CRS or grid spacing.
Scale as a sanity check
The solved scale should be close to 1.0. A scale far from 1 is a red flag — often a units mismatch between the grid and the control points. SiteLens surfaces the scale prominently so you can catch this immediately.
Next: Importing Points.