The position shown is the one your browser is willing to give, and it is not always accurate. On a phone, one setting deliberately degrades it: iOS — Settings → Privacy & Security → Location Services → your browser → "Precise Location"; Android — Settings → Location → Location services → "Google Location Accuracy". With it turned off, the phone gives only an area several kilometres across. The line above the map always shows the reported accuracy: that is what tells you how much the blue dot is worth.

Rouen Buses

Choose a route and the map shows its path and its stops. The Where am I button looks for your position and gives, in each direction, the nearest stop and the next departures.