Aligning a map image

Why alignment is needed

A scanned or photographed map doesn't know where it is in the world. Alignment (georeferencing) tells Dibber how your image maps onto the real terrain, so the map can be overlaid on the base map, control points can be detected on it, and exports can combine your route with the map image.

Placing point pairs by hand

1. In your project, complete the Map Image task (upload the scan/photo/PDF — see importing files). Importing a route first is optional but gives you a third reference to check against.
2. Open the alignment workspace. You'll see the OpenStreetMap base map beside your image.
3. Add matching point pairs: click a clearly recognizable feature (path junction, building corner, summit, bridge) on the base map, then click the same feature on your image.
4. Repeat until the overlay appears and the “Needs alignment” status clears. That needs at least 3 complete pairs and a mean residual of 4 m or less — three pairs on their own are not enough if they fit badly.
5. Select a letter (A, B, C…) to drag that point to a better spot, or remove a point you mis-clicked. Use the transparency slider to compare the overlay against the base map.

Suggest placement

Don't want to find the first points yourself? Suggest placement proposes them for you. Nothing runs on its own — you trigger it, and it works like this:

1. Press Suggest placement next to the alignment controls. The suggestion runs in the background and you can cancel it while it works.
2. If it can suggest points, you get an editable draft of three pairs (A, B, C) — not a finished alignment. Drag or remove points like any hand-made pair. Each suggested pair names what it matched — a control circle, start or finish symbol on your printed course, or a place named on the map — shown beside A, B, C; check every point yourself. Dibber never invents points it cannot tie to a feature you can see on both maps.
3. Accept when the points look right — that is what uses your suggested-placement allowance — or Discard to leave the image, your pairs and your alignment exactly as they were.

When the signals behind a suggestion disagree or are too weak, Dibber says so instead of guessing: it may show a suggested area on the base map — “the best available estimate” — for you to place A, B, C inside yourself, or a message explaining why nothing can be suggested. Either way the manual pair flow above stays fully available.

How many suggestions you get

• Free: 1 accepted placement per calendar month per account.
• Plus: unlimited accepted placements.
• Guest sessions: 1 accepted placement per guest session.
• Suggested placements consume no map-text recognition credits — the map-text recognition credits shown elsewhere are a separate allowance.
• Manual alignment is unlimited, whatever your plan.

Photos taken at an angle

Photos taken at an angle fit worse than flat scans. Before aligning, use Straighten map image (and Correct perspective skew for keystone distortion) in the map image panel. Dibber shows you beforehand whether your placement points carry over to the straightened image or are cleared, and an uncorrectable image is reported honestly instead of being forced into a fit.

Getting a good fit

• Spread points across the whole map — corners and edges, not a cluster in one area. Clustering makes the fit accurate in one spot and wrong everywhere else.
• Watch the per-point residual readout: it shows how far each pair is from the computed fit. A much larger residual than the others usually means a mis-clicked pair — fix or delete it.
• More pairs than the minimum of 3 generally improves accuracy.

What happens after aligning

The alignment is saved with the project, so it survives reloads. An aligned map unlocks automatic control-point detection and image-based exports. Removing the pairs removes the placement — there is only one way to undo.