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Old 10-07-13, 10:19 AM   #1
eja_ bottecchia
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Do high winds affect Garmin's altitude readings?

Last Saturday we experienced a Santa Ana winds "event."

http://en.wikipedia.org/wiki/Santa_Ana_winds

I went out riding and encountered very strong wind gusts. Some of the gusts, especially the gusts coming down Potrero Road were strong enough to temporarily stop my forward movement.

When I downloaded my Garmin date (Garmin 510) it showed an elevation gain of over 3,100 feet.

On Sunday I repeated the same ride, taking the exact same course. The Garmin date for Sunday's ride showed an elevation gain of less than 1,200 feet.

The only difference between the two days were the strong wind gusts on Saturday's ride.

BTW, I do that ride on a regular basis and the elevation results are always the same.

My question(s) then is: do strong wind gusts affect elevation readings and, if so, then why?

Thanks.

Last edited by eja_ bottecchia; 10-07-13 at 01:05 PM. Reason: correct elevation gain for Saturday's ride
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Old 10-07-13, 10:39 AM   #2
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Yes.

Most modern Garmin Edge devices (the 510 included) use barometric pressure for determining elevation. Wind gusts cause pressure changes (edit: or, more likely, pressure changes cause wind gusts) which convince the Edge that the altitude is changing.

I don't know whether or not that's entirely responsible for what you're seeing, but I think it is certainly plausible.

Last edited by cplager; 10-07-13 at 12:39 PM.
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Old 10-07-13, 11:12 AM   #3
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Of course, simply riding your bike normally causes winds that vary from 0 to 35 mph or so (if you've got some hills around or are really fast.) That doesn't cause strange readings in any GPS I've used, though I've not used the 510.

Even if you're also throwing in gusting "real" winds I wouldn't expect much of a difference.

HOWEVER, strong winds are caused by air moving from a high pressure zone to a low pressure zone (really, all winds are, but weaker gusting winds are often locally caused by thermals), and tend to indicate changing weather. So it may very well be that the barometric pressure changed considerably during your ride, that the pressure dropped and that was seen by the barometer as you gaining 600 feet more altitude than you really did.

To be sure, you might be able to look up the barometer readings for that day, if they're available on an hour by hour basis, and see if it was going down? You could also download your data to a computer, and compare the recorded altitude to the altitude for a given location as given by a topographic map. (This one might require some work.)
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Old 10-07-13, 11:15 AM   #4
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I had really weird altitude readings the other day in a thunderstorm. The position tracks were ok, just the altitude was off.
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Old 10-07-13, 11:51 AM   #5
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The change in barometric pressure may well explain the higher readings. May also help explain the pounding sinus headache I experience during parts of the ride.

I did not think that all of the sudden parts of California had dropped 600 feet in altitude from Saturday to Sunday. I would have felt that.
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Old 10-07-13, 12:13 PM   #6
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Originally Posted by unterhausen View Post
I had really weird altitude readings the other day in a thunderstorm. The position tracks were ok, just the altitude was off.
The Garmins use barometric pressure to improve the altitude measurements.

GPS is mostly focused on horizontal positioning (GPS alone is considered poor for altitude measurements).

Also people can tolerate much larger delta errors in positioning than in altitude measurements. Put another way, 5000 feet of altitude is a much bigger deal than 5000 feet of horizontal movement is.

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The change in barometric pressure may well explain the higher readings.
The barometric pressure also changes with temperature and humidity. The Garmin 800 have a thermometer, which means it can adjust for temperature. Beyond that, the assumption (made by the device) is that the only parameter affecting barometric pressure is elevation/altitude.

Wacky/rapid changes in the weather might significantly effect the estimation of elevation. I would guess that that would have a larger impact on long climbs than it would on rolling terrain (because the gain/loss happens faster than weather-related barometric changes).

Last edited by njkayaker; 10-07-13 at 12:23 PM.
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Old 10-07-13, 01:07 PM   #7
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I went back to my Garmin Connect page. The difference in elevation gain between Saturday's ride is a lot more than I had originally thought. For Saturday (Santa Ana event) the elevation gain was 3,100 feet. For Sunday's ride (same route no Santa Ana winds) the elevation gain was only 1,150 feet.

Big difference!
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Old 10-07-13, 02:54 PM   #8
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The pressure ports are on the bottom of the unit. If the bottom is reasonably in the clear and horizontal, then it reads static pressure and wind should not affect it significantly. If the unit is tilted so that the ports sees dynamic pressure due to wind, then the altitude reading can be affected. Even with it horizontal, if there is some obstruction that traps the wind under the unit, it can cause elevations errors. Air blowing into the ports will cause an error toward lower elevation.
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Old 10-07-13, 03:17 PM   #9
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The pressure ports are on the bottom of the unit. If the bottom is reasonably in the clear and horizontal, then it reads static pressure and wind should not affect it significantly. If the unit is tilted so that the ports sees dynamic pressure due to wind, then the altitude reading can be affected. Even with it horizontal, if there is some obstruction that traps the wind under the unit, it can cause elevations errors. Air blowing into the ports will cause an error toward lower elevation.
Interesting. Mine is mounted on a K-edge, right ahead of the handlebar/stem. It is in fairly horizontal position.

Upon additional consideration I think that the rushing winds, generated by the wind gusts, may have created some form of a vacuum which caused the barometric pressure to drop and tricked the Garmin into thinking that I was climbing higher than I actually did.

I will write to Garmin and let them know about this. Maybe they will send me a gift for noticing this glitch.

I wonder if other riders, who ride in wind gust conditions, have noticed a similar result?
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Old 10-07-13, 06:48 PM   #10
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according to my Garmin, my house was 250 feet in elevation higher when I came home than it was when I left. There is also an abrupt jump in elevation right when the rain started, and a drop when the rain ended. I could probably figure out why that was, but it would be too much akin to work, so I'm going to skip it.
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Old 10-07-13, 09:59 PM   #11
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Quote:
Originally Posted by unterhausen View Post
according to my Garmin, my house was 250 feet in elevation higher when I came home than it was when I left. There is also an abrupt jump in elevation right when the rain started, and a drop when the rain ended. I could probably figure out why that was, but it would be too much akin to work, so I'm going to skip it.
Your house rises and sinks along with the fortunes of Penn State.

That does it, I am getting a barometer to carry with me during bike rides.
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Old 10-07-13, 10:50 PM   #12
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That does it, I am getting a barometer to carry with me during bike rides.
You already have one - it's inside your Garmin GPS. Why do you think a second one will give you any additional information?
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Old 10-08-13, 12:20 AM   #13
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You already have one - it's inside your Garmin GPS. Why do you think a second one will give you any additional information?
So I can track changes in barometric pressure...the weather changes so much in SoCal.

Actually I am being somewhat facetious.
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Old 10-08-13, 05:07 PM   #14
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Wouldn't this cause problems with high pressure and low pressure weather systems? It seems to me that using barometric pressure is a much better predictor of weather than elevation. Why would his elevation always be the same unless (s)he is always riding in the same weather? I'm thinking a low pressure system that brought the winds in brought it a lot of cloud cover which substantially reduced the accuracy of the GPS.

EDIT: Changed be to me

Chris
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Old 10-09-13, 07:25 AM   #15
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FWIW: Aircraft use baro pressure to tell altitude. They have a knob to adjust for local barometric pressure as reported by nearby ground stations. Not perfect, but works acceptably well for that purpose.

http://en.wikipedia.org/wiki/Altimeter

There're not a lot of options. Radar altimeters measure distance above the ground, which never changes for a road bike. GPS altitude is relatively imprecise in that it's not good for small changes and rates of change as needed for cycling. So you're left with baro.
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