Why do Garmin and SportTracks come up with different measurements?

The short answer is I don't know. I've been trying to figure out whether to use Garmin Training Centre or SportTracks software to schedule my training and analyse data from my newly purchased 205 so have been downloading data from runs onto both programs.

I noticed a difference in pace and indeed in distance ran when I looked at my Tuesday night tempo run data across both. Curious. Garmin had me running faster splits and for a full 8 miles whereas SportTracks had me running slower splits and for less than 8 miles. The difference was pretty significant for some splits e.g. sub 7 min/mile (Garmin) compared to 7:16 min/mile (SportTracks). Why would this be?

How would distance and pace be calculated? Probably using some variant of the high school physics equation speed = distance / time I'd imagine. I'd imagine the process is something like this - the 205 pings satellites every second or so to record where I am. By interpolating the distance in a straight line between every location (co-ordinates) recorded and dividing this by time the 205 could work out pace. When downloaded to the software the average pace per mile could be worked out easily along with other info. I'd imagine these calculations to be independent of the software employed. But apparently not so.

Here are the distances and total times taken for the runs I've used my 205 on so far:



Green indicates the largest distance or time measurement. In only one run did both Garmin and SportTracks agree, and that was for total time taken. Why on earth would the time measurement vary between software? Really don't know that one.

A curious thing is that there is no obvious pattern to the variation. Garmin measures higher distances and times sometimes whilst SportTracks other times. If anyone has a clue why I'd like to know - please post a comment.

The only reason I can think of is that the way Garmin interpolates between GPS co-ordinates taken during a run and stored on the 205 is not the same as the way SportTracks does. But this doesn't explain why total run time would vary. Hmm.

Does any of this matter?

For long runs, no. Consistent pace and total mileage matter but a few seconds here or there is irrelevant.

For tempo and interval runs yes it does matter. I use the 205 to set a mile alert and to automatically lap at 1 mile intervals during tempo runs. I adjust my pace according the 205 readout and structure the whole run according to when I hear the mile alerts. For interval runs you run fast for set distance or time then run slow. If you use software (i.e. SportTracks) which does not divide your run into efforts (by distance or time) the same way that the 205 measures them during the run you cannot properly adjust your performance mid-run (i.e. speed up or slow down if you are off target pace).

What will I do? I will use the 205 and SportTracks for all runs (long and easy) except for tempo and intervals where I will use the Garmin Training Centre. Bit of a pain that I'll have to straddle 2 bits of software but c'est la vie I guess unless anyone can suggest a fix?