I believe that's the entire reason this product was built:

> OwnTracks is open-source and uses open protocols for communication so you can be sure your data stays secure and private.

I think OwnTracks was named because you "own" all of the data.

Except for displaying your location on Google Maps and using Google's APIs to do reverse geocoding...

Their definition of "private" doesn't seem to include "private from Google".

That is, indeed, the "Own" in OwnTracks, and aside from the reverse geo-coding (which has to come from somewhere!) you do own your data.

The Google Maps timeline doesn't have an API, so while you can see what data Google have there's no way to get it out and make use of it in other ways.

Google Maps Timeline has both an "export this day to KML" feature, and a "download a copy of all you data" feature via Google Takeout.

But it doesn't have an API giving anything between "export a single day" and "give me all the data ever".