Can’t Trust My Garmin

29 May

In a previous entry, I noted that my CicloMaster broke and I had replaced it with my Garmin 205. To be more precise, I have had my Garmin for more than a year and a half, intending to replace the CicloMaster. I thought the all-digital, satellite-based Garmin would be more accurate. I was wrong.

The CicloMaster is part analog. It measures distance by knowing the size of my front wheel and tracking the revolutions of a small disc attached to one of my bike’s spokes. Pretty much the same technology I had on my Schwinn Continental ten-speed back in the 6th grade. I figured 21st century technology had to be better. As I said: Wrong.

Until the CicloMaster sputtered and died, I had both units on my Torelli road bike. I immediately noticed a discrepancy between the units on my rides. After a little more than 10 miles the Garmin would show a .01 extra mile. That is, it was off the mark by just under 1%.

I had more faith in the CicloMaster because some of the bike rides I take I also have driven in my and my wife’s cars and it matched their odometers. Later I learned that Garmin devices have been tested as inaccurate.

Even before I noticed the discrepancy in distance I had discovered that the Garmin’s elevation estimates were way off. Like most bicyclists, I have many different routes, but because I ride so often, I take them repeatedly. One takes me from my house, over Skyline Road, and around Ankeny Wildlife Refuge. It’s a great ride.

On any given day the Garmin will report that I am leaving my house at an elevation of as little as 469 feet and as much as 530 feet. The top of Skyline can be between as low as 938 feet or as high as 1001. And there’s no relation between starting low at home and topping Skyline at a higher elevation. For example, the log Garmin automatically keeps indicates when Skyline was 1001 feet in elevation, I had left my house at precisely 500 feet.

I’m not sure what the problem is. It could be that the Garmin is calculating GPS data from different satellites each time, which causes the problem. Could be that, as a low-end unit in the Garmin line, the company used a less precise GPS microprocessor. Maybe Garmin engineers can’t write decent GPS software. All I know is, that the device is not accurate.

That’s a shame. Not so much for the distance or elevation data. But the Garmin unit also calculates the calories I burn on each ride. I depend on it to tell me how many of those I consume in order to calculate how many Bridgeport IPAs I can drink “for free” that evening. If get a beer belly I’m gonna blame Garmin.


One Response to “Can’t Trust My Garmin”


  1. Cheap GPS Hardware Beats iPhone Apps by a Mile | CyclistWriter - May 11, 2011

    […] my Garmin 205, a low-end GPS unit. It has six real buttons I can push that instantly respond to the command, […]

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