How reliable and accurate are GPS watches/map my run?

Dot start your run until the satellite accuracy is within 20ft. If you start I before that, the first mile can be up to +- .2 miles off in my experience. When the watch is within that range, looking at the route on google earth shows it is never far off from what you run.

Basic trigonometry will show you that a hilly course will have little effect on the "true" distance you ran. Try out the calculations yourself.

So in that regard, unless you are looking to accurately map out a race to the hundredth, GPS watches are more than accurate enough.

Most people don't double check where the actual route was plotted by going on google earth. In Garmin training center click the view tab and click view in google earth. The only major deviation from your running route will be within the first mile or so. And it's not by much.

My Garmin said the Marathon I ran was about 42.53km

who knows wrote:

My Garmin said the Marathon I ran was about 42.53km

so your saying that your watch - after a 26.2 mile run - was only ~.3ish km off?

take it back and DEMAND a refund!!!!! unacceptable!!

They almost always measure a little long but they are accurate enough.

First, the course is supposed to be a minimum of 0.1% long - 42.2 meters; typically they want a little margin. That takes a 42,195m to 42,245. So, you have 285m too long; 0.67% long. The GPS yields a point the is 'randomly' around the runner (left-right; ahead/behind); on a straight course the left-right matters more. This typically increases the measured distance relative to the true distance run. It is worse when there are a lot of curves.

Now, add in that you are not likely to have run the distance-minimizing route; chalk up another 50-150m (or if running in a big mob rather than running at 2:30 pace or better when there are not people all over). I would say that an error that might be within a small fraction of a percent is pretty good.

My Garmin FR410 is within .02mi on a marked 3 mile loop I run a lot

the only 100% accurate method is to count rotations of a wheel so hop on a bike, keep it in ONE gear, and count how many pedal rotations you make, pedalling constantly. then measure how long one cycle of pedalling takes you and multiply.

You may want to zoom in on the track to see what its looking at. There are various "twilight zone" areas in GPS that due to trees, buildings, etc, cause some pretty wild swings. If you make a turn in that dead zone you notice the track will cut the transient, or the misread will have you zooming a block over, then back.

Some routes will read spot on time and time again, others will give you total goofy. For example there's a place where I would stop for water at a country club and if I leave going south out of there the GPS would track me as going about 1/4 mile down the road and back when all I did was cross the street, so that would mean my pace for that mile would be about 100 miles per hour.

If it is hilly you are looking at upwards to 1.5 miles off in a 13 mile run. THIS IS THE TRUTH!

OKAY GUYS!!!! THERE'S ONE KEY THING YOU ARE MISSING!!!

If you measure a course with your Garmin, make SURE that your race course is USATF certified, because there's no point of comparison if your course is already inaccurate.