Aerial imagery quickstart
This quick guide explains how to map via aerial imagery using the iD editor. The concepts apply to other editors too.
Finding the most recent imagery
First, it's recommended to temporarily disable the data layer to make the aerial layer more visible. click the "Map Data" button on the right (keyboard shortcut: U)
Now, open the "Background settings" using the button on the right (Keyboard shortcut: B)
You'll find multiple background maps. Try to deduce which one is the most up to date. This is often one of:
- ESRI World Imagery
- Bing Aerial Maps
- Mapbox Satellite
Look for things such as buildings appearing in one layer but not the other. If you're not sure, you can stick to the default.
You can freely use the above sources for the purposes of OSM mapping. Do not use data from copyrighted sources unless you've been given explicit permission!
Alignment
Aerial imagery is often not fully aligned with the ground.
In the same "Background Settings" menu, enable “Openstreetmap GPS Traces”. These are on-the-ground measurements recorded by volunteers.
Scroll down in the same menu, and you'll find the imagery offset controls. Move the imagery left/right/up/down until it is well aligned with the GPS traces.
Note that the alignment is local. If you move away from the area you aligned, you may need to re-align. This effect is more pronounced in non-flat areas with mountains.
Here is an example of a bad alignment:
...and an example of good alignment:
GPS accuracy
Note that even in the good example above, some GPS tracks are not well-aligned with roads, for example, the GPS trace on the middle dirt track. This might be a GPS measurement inaccuracy, or a track measured on a road which has changed since the measurement. When aligning the aerial imagery, align it with what the majority of the GPS tracks agree on.
Mapping buildings
Aerial imagery is often not completely vertical. When mapping a building, you need to outline the ground floor. In the next example, only the bottom building is properly outlined.
End
This concludes the quick start guide. You can find more info in the broader guide.