The offline imagery of the Imperial Sand Dunes from the Glamis Guide app, served as map tiles for Gaia GPS and any other app that takes a tile URL.
URL template: https://tiles.glamisguide.app/{z}/{x}/{y}.jpg
Zoom 10 to 16, 256 px JPEG, XYZ order, Web Mercator.
Coverage: longitude -115.2421 to -114.7989, latitude 32.6868 to 33.015.
Imagery: USDA NAIP via USGS The National Map, public domain; graded by Glamis Guide.