Yes Laurie it is .1% long--but at her pace if my calculations are correct that only gives her back about 14-15 seconds for a marathon--still came in too late--she needs to qualify on certified courses and will have to try again.
I am glad to see others post this topic. It underscores the painstaking accuracy with which we take this task when people trust GPS more than our measurements.
I frequently train on courses that I have measured and love my Garmin Forerunner but it is rare to get the same distance twice even in short distances. I trust it less than a guy who says "would you like to play poker for money? I have never played before."
I use my Garmin during races and qualified for Boston wearing one. They are great but only when used correctly. I suggested turning off the auto mile lap notification during a race and physically hitting each split. It doesn't matter what the Garmin says a mile is in the race--the mile is where the marker is and on a certified course it is likely more accurate than the GPS. Those splits should tell you your real pace for the given event and help get you in on time if you trained well and have a good day.