Someone already said it earlier in this thread and it’s worth repeating. Unencrypted GPS data — in other words, GPS available to the public through commercial products — was initially only accurate to +\- 30 meters. GPS has gotten better over the years. Now, in many parts of the world you can expect much better GPS accuracy depending on how many satellites you are hitting and the GPS unit you are using. However, in some places there is no accuracy at all due to intervening terrain. GPS is line of sight. I’ve been in mountainous spots, tunnels, etc. with no GPS signal update. And rate of travel also affects received GPS data updates. Some apps are better than others, and some GPS units have better receivers and antennas than others. I use Strava on a series 3 Apple Watch matched to a 6S+ iPhone. My rides track pretty well with where I physically know I’m at. Elevation occasionally varies a bit on repeated rides but only by a few feet. If you are constantly getting wildly varying elevation deviations there is probably something in the locale or equipment or software or rate of travel that is the cause. For an example of how inaccurate otherwise seemingly reliable GPS can sometimes be, my iPhone “Find Me” app often places me at my neighbors house when I’m actually in my living room. Go figure😉 Go ride, have fun, and be okay with the GPS limitations you experience!