GPXSee/pkg/maps.txt

Open Topo Map	https://a.tile.opentopomap.org/$z/$x/$y.png	17
4UMaps	http://4umaps.eu/$z/$x/$y.png	15
Open Street Map	http://tile.openstreetmap.org/$z/$x/$y.png	19
USGS Topo	https://navigator.er.usgs.gov/tiles/tcr.cgi/$z/$x/$y.png	15
USGS Imagery	https://navigator.er.usgs.gov/tiles/aerial_Imagery.cgi/$z/$x/$y	15