Edge 1030 Distance Calculation Inaccurate

Yes, the sensor does override the GPS. The sensor is more accurate due to the GPS losing the signal due to obstructions.

Another way to look at it is if you are riding in a mountainous area, the GPS would not know you are going uphill since it is just measuring horizontal from point to point. But, you are traveling a longer distance since you are going uphill. I hope I have explained that well enough to understand. Just envision a triangle. Not a big difference but there is a difference in distance.

I talked to Garmin Support about that last year.  I "believe" if a speed sensor is connected the Garmin 1030 will use that for calculating distance.  However, Live Track will use GPS.  In my experiences the GPS is more accurate than the sensor based on mile markers I have measured against.  However per above I may have had the wrong value for my tire size configured. 

I also ride a lot where the GPS loses satellite. I may be wrong, but it's my experience that once the 1030 loses and finds a satellite again, it calculates a "straight line" from the point where it lost to where it regained.  I have lost miles that way.  In that case, I would think the speed sensor would be more accurate because it is not using gps.  But, what do I know? 

Garmin Support also told me that the sensor is intended more for indoor rides.  That did not sound correct to me though....

9884656 said:

Garmin Support also told me that the sensor is intended more for indoor rides.  That did not sound correct to me though....

Unbelievable! This is complete nonsense!
You're right about your guess. 

I can't imagine why support would tell you the sensor is more for indoor rides. I live in Atlanta and we are fortunate to have the 66 mile Silver Comet trail. This trail is mostly runs through a canopy of trees. If I don't have a sensor and GPS is used for speed,  I can watch the speed jump from 3mph to 16mph. The end mileage will be off too.

Also,I'd be cautious using mile markers to determine accuracy. I've seen some mileage markers placed wrong.

I thought it was odd that I was told that.

Sorry, cheracogoff, you are not correct in saying "just measuring horizontal" .  I don't plan to debate but the gps receiver solves 3 dimensional equations many times a second based on the time differential of synchronized signals from multiple satellites - at least 4 satellites - more is better. As you climb a slope, you are slightly closer to the satellite system and this is enough to determine your height&position of the new point and thus the slope. Those calcs are based on time differentials of radio signals received from the satellites visible. (A GPS system is a wonderful tool to have available but there are limitations, for example, there was an uncertainty in vertical position of as much as 10m in the earlier generation of GPS, and about 1/10 that in horizontal accuracy normally.  That may be better today.

All we need to understand is that the 1030 should calculate relatively accurately the distance on a climb (i.e. in 3 dimensions) provided there are sufficient satellites which usually is not a problem especially if you set the receiver to use both GPS and GLONASS (takes more power though) - the GPS system is sufficient unless you are in a heavily treed area.

Thanks for the explanation. I have been under the impression that what are called Map Grade GPS receivers are the only ones that can accurately handle vertical measurements and the Garmin computers do not use this type of GPS chip due to the much higher cost. I'm sure that Garmin uses these higher grade receivers in their aviation products.

My conclusion about the cause of the distance discrepancy is that it's the Mandelbrot effect: A more accurate measurement results in a bigger number. The sensor is running continuously and, provided the circumference is set manually, gives a more accurate record of distance travelled than the GPS which is probably using one position per second (and may incorporate an element of smoothing). The GPS track length is the sum of a series of straight lines which cut corners to some extent and therefore under-measures the true distance. Perhaps, in a straight line on the level, the two forms of measurement might agree.

I also used a 705 before the 1030 and I had both units on the bike to test for any differences in data. I found no distance difference at all. Do you have a wheel sensor paired to both computers? When looking at the route ridden using Garmin connect, I did notice a few instances where the 1030 was slightly off the road compared to the 705 but it was minor and did not effect distance. I'll try it again this week to see if the results are the same.