It shouldn’t surprise me that there’s a mobile version of the London A-Z Street Atlas. There are, in fact, several, the most recent of which is the Greater London A-Z Street Map, which covers some 3,743 km2 of territory and stores all its maps—the same maps you’d get in the paper edition—on the device. (Which makes it a fairly significant download: 603 MB on iOS, 382 MB on Android.) The iOS version costs £5 and is compatible with both the iPhone and iPad. The Android version is available on Google Play and costs about the same.