Measuring vertical distance gained is kinda like measuring a coastline. It's fractal. The closer in you get, the bigger the numbers. For cycling, 1-second recording has too fine a granularity to get a reasonable number.

It's really not possible to get an exact number. You can get a solid estimate.

FWIW, I did a route today that I've done a number of times. I get elevation numbers on my 500 anywhere from 840 ft to 990 ft. The slower I ride up the hill, the bigger the number I get.