I know that the Garmin 305 uses barometric pressure in some way for the altitude. That works quite well under normal circumstances, but in good ole flat as a pancake So Fla, it leads to some unusual anomolies.

See this ride report.

https://ridewithgps.com/trips/18674 **

** I later when back and had the site fetch USGS data and it corrected the elevations.

The actual elevation profile for the entire ride should be similar to the first 17 miles of the ride... dead flat. Then a pretty sever front moved thru (and yes we got soaked.)

Note that the elevations went wacky! At one point it changed 100 ft while I was drinking coffee at Starbucks! I know I got a little caffeine high, but....

Is there any site that downloads correct the barometric pressure data based on NOAA records for the time, date and GPS location? I know from my days Air Force days that pilots would call in for an altimeter check and get the current barometric pressure to correct their altimeters.

I rather doubt it, and it seems like a lot of trouble to go through. Most riders in flat areas aren't that concerned about elevation profiles and in areas with substantial hills the anomalies you talk about tend to get hidden by the real altitude changes.

Pilots have an advantage since the place where they need the most accurate absolute altitude information is when landing - and airports have precise local barometric data to give them. Another use for altitude data is to maintain vertical separation between planes and there it's the relative altitude that's important. If a front moves through and everyone's altimeter goes up by 100' then the separation remains the same.

Note that the Garmin units do try to correct for barometric changes by slowly recalibrating based on the GPS measurement. That works fairly well as long as the weather changes are gradual (over a few hours), but can't deal with sudden storm fronts such as the one you encountered.

Thanks for a well thought out and informative answer!

I noticed in the RWGPS site to where I had the uploaded, they have a "check this box to upload USGA data. I did, and I got a much better elevation data set. Note that it only works for US areas right now.

My 305 gets very confused by tall trees and buildings. It seems to think I've climbed a mountain once I get downtown. I don't trust it's elevation data except as a very broad indicator.

I have a Garmin Oregon 550t, which is a very different unit, but probably has a few similarities under the covers. I've noticed that altitude is the least accurate metric the thing can give me. The lat and lon measurements seem to always be within 10 feet when I plot my data over a sat map, but I often descend as much as 300 feet at the beginning of my rides ... which is odd in that I live at the bottom of a hill.

So far as I know, the Garmin units have barometric and GPS altimeters. The barometer can get you to about 10 feet accuracy, in general, but goes wonky when the weather changes. On the other hand, the GPS is supposed to be accurate to about 150 feet, and that's what you get, regardless of weather. I think the best approach is to calibrate the barometer often; I know the elevation at my front door, and try to remember to check or set it at the beginning of each ride. I'm not very successful at that...

On the other hand, when I go kayaking, I ignore the altitude plot all together. The three to four foot swells are inside the margin of error, so the data is useless to me. If I were cycling in Florida, I'd probably be thinking along the same lines...

Barometric altitude is different than geographic elevation. One references air pressure and the other is absolute height referenced to a geographic reference. If you want them to be the same you need to correct for changes in barometric pressure in much the same way as airplane altimeters are corrected. Since your unit doesn't have an adjustment built in you can find the current barometric pressure either from the weather service or flight service, convert to elevation and then record it as your starting waypoint altitude. They may even do the conversion for you>