I live on a perfectly flat barrier island, Sanibel Island. The elevation of the island is variously reported as 0', 3', or 4'. Topographic maps of the island have no contour lines, none. We do have a 70' bridge, but I have never ridden over it.

I have a Garmin Edge 200 the data from which I upload to my computer, and the program calculates all kinds of stats. One such stat is that over the last 781 miles ridden, every inch of which was on Sanibel, I have had an elevation gain of 4182 feet. I assume that the random error goes into the calculation, but since the program is only interested in elevation gain it throws out the negative error and reports the positive error as gain. Does anybody have a better guess, or even (gasp) actual knowledge about this?

I guess the 35,636 calories it says I burned is spurious too. I know I'm still fat and out of shape.

I wonder if the GLONASS system would be any better if they could get their satellites into orbit?

Dunno.

BTW, are you retired? How do you like the area? A lot of folks in my area retire to the Ft. Myers area.

You might want to try plugging your data into the Garmin Connect service. You can look at your data both as supplied and with their elevation correction applied.

What may be happening it the weight of tourists is tipping the island in such a way that you are always riding slightly uphill.

I have run it both ways, with and without the correction. Very little difference.

Probably not a tourist mass problem. The population of the island is 30,000 in season, but only 6000 during the summer. ...come to think of it about half of those miles were in tourist season...hmmmm.

No Island is perfectly flat, generally the centre is the high point, and even if it appears flat, it isn't, map elevation lines are so far apart. It's probably catching the small amount of elevation that is there.

Yeah, we are retired here and I think It is the best place ever. Lots better than Loovul.

Why are you folks so proud of Kentucky Jelly? It sure tastes bland on your toast.

4182 feet over 781 miles is about 5.4 feet of elevation gain per mile. That sounds like what you'd expect, even if the altitudes are correct, right? IOW, you'd expect that in a mile, you'd go up and down a few feet.

Whether contour lines show up on a map will depend on the scale of the map. The topo quad I called up showed lines of contour --all the way up to 5 feet in spots with elevation indicators up to 6 feet. I think your question remains valid, or at least points out "issues" with positive gain only

I thought about that too. I'm not sure what the vertical resolution is. To see what you suggest it would have to resolve inches because there is not much visible variation over a mile. I think they say that the horizontal resolution for ordinary GPS units is about 10 feet. The vertical would have to be much better than that to show a few feet per mile. I still think it is the error of the system.

Consumer grade GPS is notoriously inaccurate at determining vertical elevation, sometimes on the order of 10-20 meters. Thus the elevation gained/lost figures are huge estimates. A unit in motion over a long time, while listening to a changing constellation of satellites just compounds the errors.

I found this informative:

https://gpsinformation.net/main/altitude.htm

I see what you did there.

I have a garmin Legend and I can stand in one spot and watch the elevation change 80 feet.

Barometric pressure?

Thanks, that was a useful article.

Yes, even if it isn't precise to inches, its inaccuracy is likely to introduce swings of several feet.

Check out this profile from an out-and-back ride using my older Edge 205 (correction turned off):



Except for the red sections, the elevations should match exactly (since I came back on the same roads), but you can see the inaccuracies.

Note that when I have elevation correction turned off, my elevation gain shows 4072 feet for this ride. When I turn it on, the elevation gain shows as 2380 feet.

Standing in an elevator, right????

No. Standing on flat ground.

You may also find that if your gps is recording distance traveled that although the gps is sitting in one spot after a few hours it will show that it has traveled some distance.

The Edge 200 does not have a barometric altimeter, rather it attempts to calculate elevation using the GPS satellites which is going to be inaccurate. The telling line from the article linked to above is: