I’m aware that if I use a third-party map as a kind of “overlay” on OSM to add map information, then I’m creating a derivative work.
But, is it legal to compare, say, a Google Map with an OSM map and identify differences, and then get out in the area (without the Google Map) and collect GPS tracks and other information necessary to draw the map? That is, would it be legal to use an existing map to find out where map information is missing in OSM as long as I don’t use the existing map as a reference for drawing streets, POIs, etc.?
No, you are putting a considerable effort into mapping the missing data (by going outside and properly survey the area) which will create a ‘new works’ and therefore is not copyright theft from Google. But: IANAL, this is no legal advice.
From experience I can say it’s very hard to not get inspired by the other map, small things tend to just get copied, e.g. a small POI there, and a little extra bend on the road here, or even chosing the same placement of place= nodes.
So see to it that you have more features on your map than google maps has, and don’t use gmaps for reference while drawing.
Inspiration and plagiarism are different things. Inspiration is generally considered okay (that is, to the extent that patents aren’t infringed), so that’s probably fine. But, I suspect that copying the placements of nodes and copying specific POIs might qualify as derivative work.
If you discover that, say, Google Maps has added a particular public library as a POI, and you then decide to add this POI to your OSM, then this would probably be considered a derivative work, small as it might be. But, if you realize that Google Maps categorizes public libraries as POIs and decide to tag all public libraries you’ve discovered in your own OSM area, then it’s probably inspiration.