I have a V7 watch and a V14 phone. When outdoor cycling I would get the dreaded "dashed grey line" where the GPS was not recording and then magically reappeared in a solid colored line. I ran a test where I cycled the same course over four months with three different scenarios - one with just my watch, one with my phone powered down and the third with my phone powered up. When I just wore my watch or carried my powered off phone (a total of about eighty times) I ever got a dashed grey line AND the distance recorded was the same (within .05 of a mile). For the thirty something days that I had my watch powered on, 85% of the time I had dashed grey lines and the distance recorded varied by about .7 of a mile. I know Apple says carry your phone for better accuracy but it is just not true.
If you want more accurate recording power your phone off before you start you exercise.