Anquet Maps. That was a paid for program complete with OS map data. OS is Ordnance Survey the equivalent of USGS. The OS used to sell the data for stupid money, the program, plus map data at 8 different resolutions (1:25k, 1:50k etc.) plus Gazetteer, height database supplied on a 64GB USB stick (coz it takes up 40Gb of disk) for £395 in 2015. It’s protected and encrypted etc. and needs to “phone home” to unlock the maps. But once done, will work offline.
The data price from the OS has come right down now and there is a new version which is much more “cloudy”. So you can pay between £25-£50 a year for different resolutions of data and whether you want to require a net connection to work or be able to download all the data and work offline. My map data is 3.5years old and is not going to be updated but the subscription data is updated every 3 months. So the annual subscription is cheap enough to make sense. You pay and you can access the data on as many devices as you like and it works on iOS, Android, OSX, Windows 7,8,10. The new app is different and I’ve been waiting for it to mature a bit before moving to the new version and that will happen before end of the year.
There are others available but I like this (even though it was very expensive) because I have it installed on my PC at home, PC at work, my full size laptop and my 10in dual mode tablet. Once activated it doesn’t need a net connection and it uses the official UK mapping data. Not needing a net connection and being able to have detailed maps and height info download was worth the cost.