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@HughB
It will depend on how many samples the recorder app and run app take. If the GPS recorder app is set say to record a fix every 30 seconds and the run app is doing it every 1 second then there will be a difference when all the distances between the points are added up. I think the run app will be taking a point every 1 second as that is the default setting for the GPS when it is powered on. The recorder app will see the fixes every second but only record one every 30 seconds. The finer the detail on the tracking the more the calculation will vary as there is up to 20m inaccuracy between one fix and the next. So it is possible you could stand on the same spot and over a 30 minute period adding up all the distances between the points would add up to a significant distance when in fact you have stood still. My guess is that the run app has not yet been tested enough to minimise the errors.
Its the snowflake curve effect. Basically the perimeter of a shoreline gets longer and longer the more you zoom in. So the code needs to make sure that either it sets the GPS to send a position every 10,20 or 30 seconds OR skips gps locations that are within 20m of the previous position.
https://en.wikipedia.org/wiki/Koch_snowflake