When I ran my garmin/GPS had a different distance that the official distance markers. Why is this?

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The course is certified by Athletics Canada and is a registered AIMS Marathon (Association of International Marathons).

It’s due to the tall downtown buildings and GPS has difficulty tracking exactly where you are.

This is a very common problem with GPS devices being used in large cities and is called multi-path interference. Multi-path interference is when the device does not have a clear shot or line of sight at the sky. In areas where there are many tall buildings, the source signal of the GPS device can split into more than one signal, causing incorrect readings.