Satellite imagery is one of our common source of data for mapping, however, not all imagery is exactly aligned to real world features. Every mapper should be aware of this and should consider adjusting imagery from aggregated GPS tracks or from features which we know have correct location.

Adjusting imagery offset

The usual approach is to align the imagery to GPS traces as best as possible using the Adjust imagery offset tool before making any edit.

You can also save the defined offset as a bookmark so you can access them later on, for specific places.

These offsets are stored in your local machine and only you can access them.

Using the Imagery Offset Database

The Imagery Offset Database allows a mapper not only to store imagery offsets but also to share them with the community. This crowdsourced approach for managing imagery offset is a great way to make sure that data derived from imagery have a consistent offset adjustment. This database can be accessed through a JOSM plugin.

Here is the wiki page for Imagery Offset Database and step by step tutorial with illustrations to install and use it in JOSM.

Aligning the imagery using aggregated GPS traces

• In JOSM, load the imagery layer you wish to align (for example, Mapbox or Bing).
• Load the Strava heat map or OpenStreetMap Traces layer.

• Make sure the satellite or aerial imagery aligns as best as possible with aggregated GPS sources.

• To verify the alignment check the offset at a nearby area with a very high density of GPS data and wide roads.

Store the imagery offset

To record the offset for sharing with others and later use Imagery > Store Imagery Offset. Make sure the offset imagery is on top of other imagery or you would get a warning message.

Using the stored offsets

To use the offsets stored in the database, download the area in JOSM and place the imagery on top and click on Imagery Offset DB icon to show the offsets stored in that area and select the best suitable offset corresponding to aggregate GPS traces.

Hope this database is useful for the community. Please do comment on this diary if you have any suggestions on storing and sharing the imagery offsets while mapping at different regions.