Did a nice 35-ish mile ride today - about 1/2 rail-trail and 1/2 streets - started in Will County and ended in Cook County in Illinois.

Since I had a small issue with my speedometer, and my GPS generally shorts me about 1/2%, I checked my route with 4 online route mapping sites that would also give me something I did not have - elevation gained.
VeloRoutes.org gave me 34.65 miles and 780 ft of gain.
Bikely.com gave me 34.65 miles and 305 ft of gain.
BikeRouteToaster gave me 35.04 miles and 423ft of gain.
Gmap-Pedometer.com gave me 34.64 miles and no elevation gain, just a profile.

Any comments on who's elevation data comes from the most accurate source?

Does it really matter?

Only if you want bragging rights.

the only question is if it's 300+ or 700+ feet. The distance seems clear. These are way far apart. what does your compute say? If it agrees with either of the 300 or 700 gains, I think you have it.

Well, the compute(r) don't say, because it can't. No elevation function at all, hence my attempt to get the number using these online services. While the mileage is obvious pretty accurate, the elevation climbed was all over, so the question was posted here, hoping someone else had experience with which website has the numbers closer to right.

All the sites mentioned above are using the Google Maps API to read the elevation data from Google. I haven't found out what the datasource is for Google. My only comment is that none of the websites based on Google are going to give you elevation gain accuracy that is close to what 24K datasets provide, such as DeLorme TopoUSA or Garmin Topo 24K. The problem is that the elevation points on google are too far apart. Between two points at the same elevation, you may have gone up and down, but as far as the dataset is concerned, you didn't change elevation.

The difference between the above sites is how they plot the routeline and calculate the elevation. I'd worry less about which one is more accurate and use the same site to compare climbing ride to ride.

The underlying USGS elevation data is very inaccurate. The stated accuracy for the data is typically +/- 50 feet (7 meters) and that is for when it was compiled. In most suburban-urban areas local construction further decreases the accuracies...

Like the previous poster, I would suggest comparing the sites reported elevation changes over time. They should converge to a decent (though still not terribly accurate) average.

Accurate record of elevation gain does matter, since you may want to compare yourself to the pros. I use https://ridewithgps.com/ It's easy to use, free, and has a cue sheet print out. Checking against the barometric altimetry of my Sigma ROX 9.0 recorded at 5 sec intervals, this program agrees within +/- 16 feet over a 3.4 mile climb.

Ultimately you would want to convert to metric and figure your VAM, vertical ascent per meter, and finally your relative power output in watts per kilogram. The only additional info you need is a stopwatch to clock your climb time. Use this link https://www.cyclingfitness.net/what-i...-calculate-it/ to see if you're anywhere close to Contador and the gang.