Runner 1: "The marathon distance was wrong: My Garmin GPS watch measured the marathon distance with 42.56 km! It should have been 42.2 km. My GPS watch has always measured the distance exactly for about 30 races so far".
Runner 2: "I'm not convinced that the track length is right! My Gramin Fenix 3 and friends of mine with Suunto running watches have measured all 21.50 Km. I know that GPS and GLONASS have differences."
We don't want to give you a recommendation which GPS watch measures the most accurately, because we can't do that at all. But first of all we can reassure you about the length of the SwissCityMarathon - Lucerne. Since our route is measured and certified several times by an international surveyor of the IAAF with the appropriate measuring technique (so-called Jones Counter), we can confirm that the routes correspond to the specifications and therefore have the correct distances.
Nevertheless, the recurring feedback regarding the track length made us think about that. Therefore we searched for possible reasons for the difference between the official measurement procedure and the local GPS location. We found a first hint in an article in the magazine Fit for Life, which interviewed the international surveyor John Kunkeler: "GPS watches are simply too inaccurate. They interpolate, have dropouts under trees and in canyons. We once invited 80 authors of these complaints to an information event, and in fact 25 came. Together we ran the 1000-metre reference section measured by the Civil Engineering Office. The best one came close to seven metres. Discussion over. (Source: https://www.fitforlife.ch/artikel/marathon-streckenvermessung/).
According to this practical test, the "best" watch on a marathon already shows a difference of 294m (42x7m). With this difference we are pretty much in the range of our runner 1's statement.
Thanks to further research in this area, we have come across interesting findings that explain the described deviations both theoretically and practically. Of course we don't want to withhold this from you. The following investigations from Germany give information about why the measurements with GPS clocks are inaccurate and cannot be used as a reference.
You can find the corresponding study and further information about measuring a track on the website of German Road Races.