I think that using the Arduino to help with geologic mapping is "the long way around".
Nowdays, most smartphones have built-in GPS and compasses and there are probably lots of cheap or free apps that would help you with mapping. The typical cost of a smartphone with GPS and compass is $50 to $100 new, cheaper on the used market.
A good example of archaeological mapping may be found at:
Landscape Surveying using Handheld GPS Receivers
The app "Geology Sample Collector" may be something you can use.
https://play.google.com/store/apps/deta ... 1000&hl=en
Or maybe GeDroid Mapper:
https://play.google.com/store/apps/deta ... pper&hl=en
https://gsa.confex.com/gsa/2012SE/final ... 202468.htm