edited January 24
My tomtom runner is showing distances that are lower by about 4% than what I know they should be.
How do you know what the distance should be? How are you measuring it and to what degree of accuracy does it record? Unless you are using a calibrated measuring wheel and following the line of that wheel exactly, any measurement is an estimate. The nature of GPS is that each data point collected is only accurate to within 15-30m of your actual location. The watch collects the data points and then tries to figure out a route using them (considering that some points may overlap or duplicate). The watch then smooths the data to come up with the most reasonable route taken according to the algorithm it is programmed with. This is why every watch, even two of the same model, will come up with different distances, as they are collecting different GPS data. Even on a certified course the distance is a best case scenario running 18 inches from the curb, not going around anything and taking all tangents. In addition, measured courses add 1 meter per kilometer as a buffer so they are always a bit long. Even championship marathoners actually run longer than 26.2 miles in a race.
I hope this helped answer your question. If so, please mark it as a solution so others can look for it if they have the same question.
The course I ran is a popular course which has been measured by many running groups and the TomTom is measuring 4% lower for that course. It is also measuring 4-5% lower for another popular course. I am running city streets down straight lines and not deviating in any way.
Thanks for your explanation regarding how GPS is not perfect. I can accept being off by 1% but at 4-5% I may as well be guessing how well I am running or I can just use a watch to time my course and guess how far I am running. I love the simplicity and interface TomTom provides but it is just too inaccurate for me. I am going to switch out to a free iPhone app.
Welcome and support
Maps and Services
RoadTrips by TomTom
Community Knowledge Base