I've installed Google Maps (what Apple formerly used) and find I am using it more often then Apple's mapping and routing system.
If you like Garmin or Tom Tom approaches to mapping and routing, those are both available for purchase as iOS apps, though they work best with 3g or 4g phone connections.
I've also installed City Maps to Go and a topographic maps app that maintain map data on the device, for reference when I don't have a connection. And the National Geographic World Atlas, and several collections of historical maps.
DeLorme and Microsoft have not ported their mapping applications to iOS, nor to Android or any other mobile or tablet OS since DeLorme stopped supporting the defunct PalmOS and Microsoft gave up on the Pocket PC version.
Astronomy, I like Sky Safari Lite, though I also have Stars and Planet Finder (and a dozen other astronomy apps I don't use much, like one for the Hubble and another for pulling current images from a solar telescope).
I don't try to use a tablet or smartphone as a navigation device while driving, as the displays are not really designed for the lighting evironment of an auto interior in daylight. Considering the limitations of the GPS receivers in these devices, if they even have them, and the cist of a good navigation app, I find it more cost efffective to just buy a low end dedicated GPS navigation device.
There are tens of thousands (hundreds of thousands?) of apps in the App Store. The shopping interface shows you only a few hundred of the most popular in your market, by categories. You can shop more of them using iTunes from a computer. I started out using a function called Genius on my Touch, it found 100+ Apps fitting my tastes, and I haven't bought much since the first six months on each device. Genius seemed to disappear from the iPad with one of the OS updates, maybe to iOS 7.
Tom Test
Itasca Spirit 29B