Why three points in a line will not work
Three Anchor Points along one road, one fence or one piste cannot calibrate a Map, however carefully you place them. The editor says so: collinear points can’t define a 2D transform. This page is why, and what to do instead.
Why
Calibration fits one relationship that maps your whole image onto the world — a stretch, a rotation, a scale, a skew. Three examples are exactly enough to pin that down, provided they are spread in two dimensions.
Points in a line only tell it about one dimension. Along the line, everything is determined: the scale, the direction, the offset. Across the line, nothing is. The picture could be stretched to twice the width, or flipped, or sheared into a diagonal, and every one of your three anchors would still land exactly where you said. There is no single answer, so there is no calibration.
Nearly in a line is the same problem, weaker
This is the part that actually costs people time, because there is no warning for it. Three points in a slightly-open triangle — say two ends of a main path and a third point a few metres off it — do define a transform, and the editor accepts it with a perfect score.
But the answer is fragile: the little bit of spread across the line is doing all the work of pinning down that whole direction, so a small error in the off-line point becomes a large error everywhere away from the line. The symptom is a Map that is accurate along one axis of the venue and drifts badly the further you get from it, and it will not show up in the Calibration Quality figure — diagnose “my friends show up in the wrong place” is where it surfaces.
What good spacing looks like
Think of the widest triangle you can draw inside the area you care about.
- Three anchors near three different corners of the venue. Not three along its long edge.
- The triangle should be fat, not thin. If you can draw a straight line that passes close to all three, move one.
- Add the fourth in the remaining corner, which also gives you the first honest error figure.
The editor’s own Anchor Placement Tips put it in one line: spread anchors far apart, covering the map’s corners is ideal.
The awkward case: a long, thin venue
A ski piste, a river path, a linear festival strip. Sometimes there is genuinely nothing off the line.
- Widen your definition of the venue. The car park at one end, a building set back from the path, the far side of a bridge — anything that is on your image and off the axis will do, even if nobody goes there.
- Use the ends and one side. Two anchors at opposite ends and one as far off to a side as the image allows beats three evenly spaced along the middle.
- Accept that across-axis accuracy will be poorer, and check it with the heat map rather than assuming — read the accuracy heat map.