Choose and prepare a venue image
This is the stage that decides how good the finished Map can be, and the decision is close to irreversible: a bad source image cannot be fixed by adding more Anchor Points. Ten minutes spent here saves an hour of calibration that never quite works.
The rule that matters most: the image must be flat
Calibration works by fitting a single, uniform relationship between your image and the real world. It can stretch, rotate, scale and skew the whole picture at once. What it cannot do is undo a distortion that varies across the picture — and that is exactly what a photograph of a physical map has.
So a photograph of a paper map on a table or a board is the one input to avoid. In a photograph taken from an angle, the far side of the map is compressed and the near side is stretched. You can place three Anchor Points and get a perfect fit at those three points, and everything between them will be wrong — by tens of metres, in a direction that changes as you move across the venue. Adding a fourth and fifth point does not average it out; it shows you the error and cannot remove it.
If a photograph is all you have, photograph it straight down, from as far back as you can, with the map filling the frame. Or better: find the digital original. Almost every venue map exists as a PDF somewhere.
What makes a good image
| Property | Why |
|---|---|
| Flat and straight-on | The one thing calibration cannot compensate for |
| Covers the whole area you care about | A Map stops at its edges, and people outside them are told they are off it |
| Legible at a phone’s size | The names on it are what people read while walking |
| Reasonably plain | A busy decorative map competes with the dots drawn on top of it |
| Yours to use | You are asked to confirm it, and reviewers are shown what you say about its source |
North does not have to be up. Calibration handles rotation, so a piste map drawn at whatever angle suits the mountain is fine.
What to do about a PDF
Export a single page as PNG or JPEG at a generous size — around 2000 px on the long edge is plenty. Do not screenshot it: a screenshot at screen resolution throws away the detail that makes labels legible when somebody zooms in.
Cropping
Crop to the area you want the Map to cover, plus a little margin.
- Crop off the legend, the timetable and the advertising. They take up space that the map itself could use, and they are not part of the place.
- Keep the edges you will use as Anchor Points. Cropping tightly to the interesting bit removes exactly the corners that make good, well-spread anchors.
- Do not crop after calibrating. Cropping changes where every spot sits in the image, so every Anchor Point moves and the calibration is invalidated. Crop first, once.
Size and format
The editor is forgiving here and will do most of the work:
- PNG or JPEG. Any ordinary image format the browser can read.
- Under 10 MB. Larger is refused with File size must be under 10 MB.
- At least 400 px. Smaller and the editor warns that the image may be too small for a useful map. It is a warning rather than a refusal, and it is usually right.
- Big images are optimised for you. The editor resizes and compresses on upload and tells you what it did, so you do not need to prepare a web-sized copy yourself.
A worked example
A festival gives you a site plan as an A3 PDF.
- You export page 1 as PNG at 2400 px wide. That is your source, and you keep it.
- You crop off the sponsor strip along the bottom and the legend in the corner, leaving the site itself with a small margin. 2100 px wide.
- Before uploading, you note three unambiguous features near the edges — the main gate, the north-west corner of the car park, the bridge — and look up their coordinates: what you need before you start.
- You upload it, fill in Map Name, note where the image came from under Image Source, and tick I confirm I have the rights to use this image.
Total, about fifteen minutes, and calibration then takes ten.
Had you instead photographed the printed programme at an angle, steps 1–4 would have taken five minutes and the Map would have placed people in the wrong field for the rest of the weekend.