My friends show up in the wrong place

Somebody standing at the main stage is drawn in the car park. This is the complaint a map creator gets, and calibration is usually the cause — but not always, and the four things it is not are quicker to rule out than to fix.

First, work out which kind of wrong it is

Ask the person reporting it for one more fact: is everybody wrong, or just them?

The symptom	The cause	Where to go
Everybody is offset in the same direction, by about the same amount	Calibration. All anchors are consistent and consistently wrong	Below, and check your coordinate list
Everybody is wrong, differently in different parts of the venue	Calibration. Your anchors disagree with each other	Read the accuracy heat map
One person is wrong and everybody else is right	Their phone, not your Map	Not a calibration problem — see below
Everybody is wrong by a huge distance — a different town, or the sea	One anchor’s coordinates	Below, step 2
Nobody appears at all	Not calibration	See the four non-causes

The four things that are not calibration

Rule these out before touching an Anchor Point, because each is common and none is fixed by recalibrating:

1. They are on a different Map Version. If you published a corrected version, everybody who installed the old one still has the old calibration until they update. That is by design — the older Map Version keeps working — and it means your view can be right while theirs is wrong. See what happens to people on the old version.
2. Their phone has no good Position. A phone indoors or with location switched off reports nothing, or reports the last thing it had. The person’s own app tells them which; from your side it looks exactly like a bad map.
3. They are genuinely outside the image. A Map stops at its edges, and the app says so rather than drawing them at the boundary. If your image crops off the overflow car park, people in it are not placeable.
4. They are looking at the right place and describing it wrongly. The car park on your Area labels may not be the car park they mean. Check the Area names before the geometry.

Reproduce it in the editor, before you go anywhere

You do not have to be at the venue to check a report. Beside Heat map on the Calibration tab is GPS preview, and it answers the complaint directly: give it a coordinate, and it shows you where your Map puts it.

1. Switch on GPS preview. Like the heat map it is disabled until you have three Anchor Points, and the toolbar says Needs 3+ Anchor Points until you do.
2. Under GPS test points, type the Latitude and Longitude of the place the person says they were standing — the main stage, the gate, their pitch — and use Project GPS →.
3. A test point is drawn on the image where your Map computes that coordinate to be, and the sidebar lists it with the pixel position it was given. If it lands where that place actually is on your picture, your calibration is right there and the fault is one of the four non-causes above. If it lands in the car park, you have reproduced the complaint on your own screen — and you can retry it after every correction instead of waiting for somebody to walk past.
4. A coordinate that falls outside your image is still shown: it is pulled to the nearest edge, labelled ⚠ on the picture and tagged out of bounds in the list. That is non-cause 3 confirmed rather than guessed.

Two or three test points spread across the site sort the fault into its kind without asking anybody else anything: all of them wrong in the same direction is step 3 below, some right and some wrong is step 2. Clear all removes them. Test points are a check on your own screen — they are never part of the Map and nobody using it sees them.

Then fix the calibration

1. Open the Map’s Calibration tab and read the figure. Over 10 px is High error and the editor says to check the heat map. Three px or less is Good — and if it is Good and people are still misplaced, your anchors agree with each other and are wrong together, which is step 3.
2. Switch on the heat map and find the intense region. One warm area is one bad anchor: check its latitude and longitude against your list, in that order, with the sign. Latitude and longitude swapped, or one digit transposed, is the single commonest fault and it is invisible in the figure until you have four points.
3. If the figure is Good but everybody is offset the same way, check every anchor’s coordinates as a block. A mistake made once in every anchor — the wrong sign, or coordinates taken from a venue’s label rather than its gates — produces perfect internal agreement and a Map that is uniformly wrong. Nothing in the editor can detect this. The check is your list.
4. Check your anchors are not nearly in a line. Three or four points strung along one road define the transform badly even when they do not define it impossibly, and the result is a Map that is accurate along that road and drifts off it. The editor warns that collinear points can’t define a 2D transform; near-collinear is the same problem, weaker. Add a point well off the line — why three points in a line will not work.
5. Check the source image, if nothing above explains it. Intensity that varies smoothly across the whole picture, with no single culprit, means the image is not a uniform view of the place — a photograph taken at an angle, or a schematic that is not to scale. More anchors will not fix it; a better image will.

Publishing the fix

If the Map is still unpublished, correct the anchors and you are done — your changes are live for anybody who has been given access.

If it is published, you cannot edit it. The correction goes into a new Map Version, and people choose whether to take it:

• A published map cannot be edited
• Create a new Map Version

Calibration corrections are the commonest reason to publish a new Map Version, and worth doing promptly: until people update, they are looking at the fault you have already fixed.

A worked example

A festival reports that everybody at the main stage appears about forty metres north.

1. You ask whether it is everybody. It is, consistently, in the same direction.
2. The figure reads 1.9 px — Good. So your anchors agree, and step 3 applies rather than step 2.
3. You check the list. All four latitudes came from the mapping site’s label for the venue rather than from the features you clicked, and the label sits at the site office, forty metres north of the gate you used as your first anchor.
4. You re-derive all four from the features themselves, replace them, and the figure reads 2.2 px. You walk the site and people are where they are standing.
5. The Map is published, so you create a new Map Version and publish it, and tell the organiser to ask people to update.

Note what did not find this: the figure, the heat map, and adding a fifth anchor. A fault made identically in every anchor is only visible in the source list.