Open Source GPS Software

Viking is a free/open source program to manage GPS data. You can import, plot and create tracks, routes and waypoints, show OSM, Bing Aerial and other maps, geotag images, create routes using OSRM, see real-time GPS position (not in Windows), make maps using Mapnik (not in Windows), control items, etc. It is written in mostly in C (with some C++) & the GTK+3 toolkit. Viking is currently dual managed - with a corresponding Github presence at: https://github.com/viking-gps/viking Where you may find it easier to view the code and raise issues there too.

XCSoar is a tactical glide computer for Android, Linux, macOS, and Windows.

Karto allows to calibrate a scanned map (or aerial photos) with some known geo-points (Lambert, UTM, ...). After that, you can add any points (GPS points) in the map to build your own map. You can also add a transparent GIF image in the map after calibration (cave, mines, roads, ...). You can realize your own cartes with the data you add inside adding/showing/hiding layers of data. You can import/export your data files (after checking and correcting them) in differents formats.