Maps to dowload to iPad

I have just installed iNavX with sea maps in HD, on the iPad, and wonder if it is possible to install something like Google map too, so it will work when there is no internet or external connections. Like a car navigator?
I have found National Geographic World Atlas HD for iPad, and it might be close to what I am looking for, but I wonder if I should look at other apps before I buy this from National Geographic?

Thanks for the tips on a good map I will also check them out.

I was in Trondheim the first week of September and it is a pretty little port,much bigger now than my first visit in 1951.

Al.