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WHY DON'T MILE MARKERS MATCH UP WITH MY GPS?

WHY DON'T MILE MARKERS MATCH UP WITH MY GPS?

I am a 21-year-old female runner, and I have completed two half-marathons and two full marathons. I am a PR-seeker and eventually would like to qualify for the Boston Marathon. One of my biggest problems with achieving a PR during a race has to do with discrepancies between my GPS watch and the race-course measurements.