I read an article in a magazine or online about a (.gov, I think) site which showed two, four to six character sequences in a mobile phone browser window. The letters from left to right indicated lower to higher precision positioning. As precision decreased, the characters on the right turned red, indicating a best guess accuracy.
There wasn't an app to install. It was a web page where you would visit it once while on 4G/WiFi, let it download some data to your phone (I think it was an HTML5 web app), then it could be used offline, so long as you had a GPS signal.
I'm going on a trip soon and I'd like to use this app, but I forget what the website was. I saw it within the last year. Does anyone have any clue what the site could be? It could have used maidenhead or MGRS, but I'm not 100% sure.