What's your GPS experience?

[jischr]

I just picked up a Lezyne Super GPS and went for my first ride. Surprising to me it lost roughly 2 mph compare to the wheel/magnetic computer when under over hanging tree cover or even tall trees on one side. The signal strength stayed the same at 4 dots throughout the ride. The Lezyne FAQs admit to some loss of accuracy under 'heavy tree cover'. If I can see blue sky, or gray sky in this instance, I don't consider it heavy.
Does your GPS also lose accuracy? How much? When your unit comes out from under the trees does it recalculate your position and time and adjust its memory? It doesn't appear the Lezyne does since the avg speed was about 1.5 mph slower than the mag computer, and hence about 1.5 miles different in distance. I suspect at most 0.5 mph could be due to tyre diameter being different than what I originally loaded into the computer some years ago.
What's the value of GPS in the treed states if its going to be wrong for 30-40% of a ride? This could turn into a rather expensive heart rate monitor.

[kramnnim]

I just use an ANT+ speed sensor in combination with the Garmin head unit. Never noticed any loss of signal when riding under trees, though.

[lhuerta]

+1

U will need to get a speed sensor

[Ralph]

My Garmin Edge 20 uses the US and Russian Global positioning satellites (GLONASS can be turned off). Never noticed any issues with over head trees.

[unterhausen]

just because you see blue sky doesn't mean there are enough satellites visible to give you a good position estimate. Minimum of 3, but 5 is more like it. I kinda wish that the garmin wouldn't be quite as obnoxious when it loses satellite contact, but I'm used to it. It happens in some places that I wouldn't necessarily expect, as you say.


I have no idea how they would continue to give you speed estimates when that happens, a couple of miles per hour seems really good

[makoti]

I live around a lot of wooded areas. Rides through the tress must have a sensor or the reading are junk. Using a 500, so newer versions may (should?) be better. Usually, clear skies gave me a decent reading, but not always.

[sandyrs]

If you don't need the benefits of a GPS unit besides measuring your speed (navigation, data access, etc.) maybe you don't want one after all.

[benb]

You sure it's not reporting average speed as opposed to moving average speed? That seems more likely than signal problems.

With no wheel speed sensor you are likely to see incorrect instantaneous speed readings, with the error getting worse as tree cover increases and ascent/descent increases.

But you should see OK distances unless something is wrong or the Lezyne has really poor performance, road cycling is not a particularly hard test for a GPS.

Take a look at your track in the computer. Does it show you cutting through buildings and through yards? Zig-zagging across the road? Or nice lines showing you on the correct side of the road? If the track looks really really erratic then you would see distance errors.

I have 3 devices... an eTrex Vista HCx (these were supposedly more accurate than more modern Gamin devices), a Fenix 3 (watch form factor) and an Edge 1000. The Fenix 3 definitely has worse performance than the other two but for road biking and even MTB (if the watch is on the bar) the track are pretty good. For the other two devices (bigger antennas) the tracks are super clean.

Trail runners and hikers often see the worse tracks.. those are the super hard tests of GPS distance measuring.