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· haole from the mainland
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kdub said:
The altitude reading on the 305 is pretty accurate. Because the elevation sign on the mountain is prettty much the same as what the Garmin tells me. Perhaps +/- 2-5 feet variance only.
My Forerunner 305 is often less accurate than that. For example, telling me I'm at 80ft elevation when I'm at maybe 10ft above sea level. Because of that I don't trust the instantaneous % grade estimates either.
 

· Cannot bench own weight
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4,298 Posts
Not so sure about the actual altitude measurement, but judging by how wildly ascent and descent varies from day to day on the exact same route, I'd say it's not all that accurate.

I have a 31 mile loop that I do often, and I've seen ascent total around 1200, and I've seen it around 2500, and everything in between. Same route, just a different day.

The GPS however, is scary accurate as you might expect. I hit the 31.0 mark at the exact same stoplight, every single time. I don't even use the cadence/speed sensors on one of my bikes because I trust the overall speed that much.
 

· Frog Whisperer
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is the 305 barometric?...or does it go by gps?....if barometric....it either needs to be calibrated to compensate for barometric pressure, or be "self calibrating"

If it goes by gps....then reflected "noise" can seriously interfere with elevation readings
 

· haole from the mainland
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GPS. Three satellites are needed for lat/long, four to get elevation as well. Could explain the error differences between distance and elevation.

I also find the GPS distance estimates to be very accurate. I often ride along part of the Honolulu Marathon route, where there are marked miles painted on the road. I find my GPS to be spot on with the markers, right down to a hundredth of a mile.
 

· Steaming piles of opinion
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Touch0Gray said:
is the 305 barometric?...or does it go by gps?....if barometric....it either needs to be calibrated to compensate for barometric pressure, or be "self calibrating"

If it goes by gps....then reflected "noise" can seriously interfere with elevation readings
Both. Not sure of the exact operational details, but it does use gps to calibrate the barometer on startup.

But really, calibration isn't all that important for it's intended use. After all, the useful bit isn't that I'm 603 ft above MSL; it's how much climb or descent I make as I ride.

I suspect that ride log data (which is basic gpx code) is reading off the gps; hence the varying total diffs from day to day. I believe the instantaneous grade/alt data on screen is barometric. Once it settles in, it doesn't bop around as much as typical GPS altitude data does.
 

· Frog Whisperer
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I ride with a garmin legend...distance is not an "estimate" it is right on the money most of the time. I log in to 7 to 10 satellites at most times and get an accuracy of 5 to 7 feet generally.

my elevation, if not terribly accurate, is at least consistant
 

· Big is relative
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I commute with my Garmin each day. The high point of my commute can vary by as much as 100 feet from the morning to the afternoon. Friday it was 432 feet in the morning and 512 in the afternoon. My route never changes but the total elevation gain can vary by as much as 300 feet.

I did a century a few weekends back, my elevation was dead on with the route map. Single rides seem ok for acuracy.
 

· Frog Whisperer
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if the commute is in and area with buildings or hills there is a possibility of what has been called the multipath effect. This can confuse the accuracy of the gps
 
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