We are currently using Google Maps embedded in an iOS app, however the resolution in some areas (USA/Canada) are not good enough to be able to take measurements.
We are looking for an imagery solution to replace Google Maps, Google Earth could have been a solution however they recently announced that they will no longer support iOS.
Have a look at Digital Globe's map API. We partnered with them recently and I learned about their mapping disaster recovery assistance in Nepal after the quake. I think the maps they offer will meet your needs: http://developer.digitalglobe.com/maps-api/