Place your first three Anchor Points

You have uploaded an image and you are on the Map’s own page, on the Calibration tab. Nothing works yet: with fewer than three Anchor Points the Map cannot place anybody, and the editor says so until you have at least 3 anchor points.

Have your coordinates ready before you start — what you need before you start. Placing anchors is five minutes; finding out mid-way that you do not have the numbers is the whole afternoon.

Pick the three spots first

On the picture, before touching anything, choose three features you can identify both on your image and in the real world:

• Unambiguous. A gate, a building corner, a path junction, a mast, a bridge. Not “the middle of the field”, which you cannot point at twice the same way.
• Spread to the edges. Near three different corners of the area you care about. Anchors bunched in the middle leave the perimeter to extrapolation, and that is where the error shows up.
• Not in a line. Three features along one road cannot define the transform, and the editor will refuse to make sense of it — why three points in a line will not work.

Think of a wide triangle covering the site. That is the shape you want.

The two ways to add one

By clicking, then correcting. Click the spot on the image; an Anchor Point appears there, and you fill in its latitude and longitude. Good when you are working from the picture outwards.

By typing all four numbers. In the side panel, under Add Anchor Points, use the Add by coordinates fields: Pixel X, Pixel Y, Latitude, Longitude and Label (optional), then Add Anchor Point. Good when you have a prepared list, and it is the only way to be exact about the pixel position.

Both produce the same thing, and both live in the same panel — the clicking route is the Click the map to place an anchor point prompt directly above the coordinate fields. Most people click for the first three and type for corrections.

Do it

1. Zoom in before you click. A click that is ten pixels out on a zoomed-out view can be many metres out on the ground. Zoom until the feature is unmistakable, then click its exact point — the corner itself, not near the corner.
2. Enter the coordinates as decimal degrees, latitude first, longitude second. Around Denmark that is roughly 55-something and 12-something; if you have typed 12-something into latitude, the Map will be projecting from the Indian Ocean.
3. Give it a label — main gate, car park NW. Optional, and worth thirty seconds: in a month the labels are the only way to tell which anchor was which.
4. Repeat for the second and third, at the other corners of your triangle.
5. Read the panel. The placed Anchor Points are listed with their pixel positions and their coordinates, so you can check the pairs at a glance.

What you should see when the third one lands

The Map becomes calibrated, and the Calibration Quality figure appears as 0.0 px.

⚠️ That zero is not a result. Three points can always be fitted exactly by the transform being fitted, so the score is arithmetic rather than evidence — the editor says as much beside it. You have a working Map and no idea yet whether it is right.

The fourth point is the real test, and it takes two minutes

Place a fourth anchor somewhere well away from the first three. The figure changes from 0.0 px to something honest:

• 3 px or less — Good. Your three were consistent, and you can stop.
• 3 to 10 px — Fair. Usable. Worth a fifth point and a look at the heat map.
• More than 10 px — High error, and the editor tells you a misplaced anchor is likely. One of the four is wrong; find it with read the accuracy heat map.

Then place a fifth. The editor’s meter is built around five, and it tells you how many more to add until you get there.

Undo is there

The panel has ↩ Undo and ↪ Redo, with the usual keyboard shortcuts. Placing an anchor in the wrong spot costs nothing.

A worked example

A 2100-pixel site plan of a festival field.

1. You zoom to the main gate at the south and click the centre of the gateway. You type 55.67254 and 12.56571, label it main gate, and save.
2. You zoom to the north-west corner of the car park and click the fence corner. 55.67491, 12.56102, label car park NW.
3. You zoom to the bridge in the east and click the middle of the span. 55.67108, 12.56890, label bridge.
4. The figure reads 0.0 px. You place a fourth at the south-east gate: 2.4 px, Good.
5. A fifth near the centre: 2.9 px. The meter fills and the hint says the Map is well calibrated.

Total: about eight minutes, because the coordinates were already written down.

Next

• Where to get the real-world coordinates for a spot
• Read the accuracy heat map
• Draw an Area, once the Map places people correctly