In preparation for my cycle trip in the Western Isles, I thought it might be an idea to get GPS sorted out on my phone. A bit of googling and querying brought me to a system called Osmand, which all sounded great until I tried getting help from the help files:

Usually you have 1 map, but you can make it transparent to show an “underlay” map, and you can put a transparent overlay map above. So the normal map is the middle map then. Samples of underlay maps are some “base maps” in the layer list, as openfietskaart base map. The normal map can also be a vector map. And on top of that you can show an overlay map with some transparency (overlay transparency). On top of the three maps you can always show the additional layers.

What proportion of the world’s nominal technological capability, I wonder, lies permanently fallow because the people who develop it fail to grasp that what can’t be accessed might just as well not exist?