Garmin / MB differences (Read 836 times)

I noticed that with Sports Tracks. My engineer husband says the programs might use a different algorithm than Garmin does. It could be correcting for elevation, but it sounds more like on your hike you lost the satellite. Look at the dots on your Garmin plotting and see if there are some spaces. (I am just learning about this, too)

Is it correcting for elevation change or some other effect I'm not aware of?

No. And if it were, it would not make much difference. I have previously posted the following: --------------------------------------------------------------------------------------------------------------------------- Doing the math: Grade is defined as rise over run, multiplied by 100. So a course that rises 100 feet over a mile is (100 / 5280) * 100 = 1.8% To determine the distance run on the hypotenuse of this route, you can apply the Pythagorean Theorum, which states that A^2 + B^2 = C^2 (where the notation ^2 means squared). So in a triangle formed on one side by the distance 5280 feet and on another side by the distance 100 feet, your hypotenuese = sqrt(5280^2 + 100^2) = 5 280.9. Catch that? A 2% grade, which is just over a 100 foot climb over a mile adds one foot to your distance. That is less than 0.1% error per mile, which is insignificant. That is less than a foot of error per mile. This may matter more on a nasty run, such as the Pikes Peak Marathon. The Pikes Peak Marathon includes a 13 mile climb up more than 7500 feet, with an average 11% grade. Assuming you actually do climb 13 miles in 7500 feet, let's determine the error. 13 * 5280 = 68 640 feet. Applying the Pythagorean Theorum, we get sqrt(68 640^2 + 7500^2) = 69 048.5 feet. That is a difference of 408.5 feet over 13 miles (31 feet per mile), or a 0.6% error. Or for a steady 10% grade over a mile: A = 5280 feet B = 0.1 * A = 528 feet C = sqrt((5280^2) + (528^2)) = 5306 feet So, by climbing a 10% grade for a whole horizontal gmap mile, you have added 26 feet. That is an additional 0.4%. Not worth it to me.

When I experimented it seemed like in places where the garmin lost signal, MB drew a different path for the missing signal...It appeared that since MB uploaded street maps, it tried to follow them. The MB seemed slightly more accurate. In areas that are not wooded or tall buildings where the garmin works well, the difference between the two is minimal.

I think it must have been a signal dropout now. I was carrying a hand held Garmin eTrex (my GF was wearing the Forerunner) and I was able to pull out the track file from the memory and upload it to MotionBased. MB showed this file as having a distance of 8.9 mi, much closer to what the Forerunner showed at the end of the hike. Looking at the track there is only one obvious break, and it is pretty short so I don't think it would make up a full mile. What I believe did happen however, was a huge miscalculation in elevation at that point. The elevation graph for the Forerunner track shows a gigantic spike there, while the eTrex track is nice and smooth. Also, the Forerunner track lists a change in elevation of almost 7000 feet, while the eTrex track shows about 1800, which sounds about right considering the hike.

Thanks for the info, jeff.