With the app Rally Rec, you can measure easily distances during your Rally Recces!
This app allows you to measure distances within one meter, by only using the GPS signal of your mobile (not telephone network nor internet 3G/4G).
GPS tracking works great on mountains and countryside roads. Measurement errors lower than 1%.
A memory mode allows you to memorize the last measured distance.
Can be used with a Bluetooth button to make it easier to reset the partial counter.
Installation guide :
http://www.rallyrec.com