I’m trying to improve the accuracy of road layouts in my area but I’ve found places where some features seem to have been drawn over one satellite image, but other, connected features relate to another satellite image which is slightly offset from the other. What’s the best way to resolve this? I started manually realigning everything to a single satellite image, but I expect I’ll run into the same problem when I find other features which were drawn to yet another satellite image.
This is very common, and somewhat unnerving at the outset.
The simplest approach is make any available GPS traces available (its on one of the layers menus in iD). If there are enough GPS traces these can be used as a guide to identify which imagery layer is well aligned with on-the-ground features. Where I am this is often the Bing layer, but it is also often the oldest. I therefore sometimes draw new things using a more up-to-date layer and then move them (sometimes I draw a guide way showing the offset between layers to help with this). I would also expect the ESRI layer to pretty accurately aligned and any national imagery if available.
A more sophisticated approach is to again use GPS traces to create imagery offsets of each layer. The editor should then apply these offsets and all the imagery layers should align (at least on ground level features, building parallax will depend on relationship to the camera).
If there are no GPS traces it gets harder. It is definitely useful to create some & load them up to OSM. There are plenty of Android apps (e.g., OSMTracker, OSM And, etc) which allows one to do this.
Also don’t worry over much about this: it is more important to ensure objects maintain a correct topological relationship than that they are precisely accurate positioned in relation to each other. In most cases the discrepancies are only a few metres and neither gadgets using the data nor general mapping is going to be much more precise.
+1 to everything that SK53 has said.
One other thing to mention - depending on where you are in the world there may be other available layers that you can use to help with alignment if there aren’t any local GPS traces. If you’re i the UK, then the OS OpenData StreetView data that’s available within OSM editors can be useful. It’s pretty good for road alignment (although some other features in there are less good - it’s difficult to infer some minor road classes, for example). If you’re somewhere that accurate building footprint data is available then that might be useful - ask mappers near you what layers they trust locally.