I’ve had the same issue as @KHV (and, I suspect, many others), and it’s due to the issue that the GPS uses the WGS84 ellipsoid model to calculate sea level (and therefore altitude), rather than the Geoid model. The Geoid model provides the correct sea level (and therefore the correct altitude), but its difference relative to the ellipsoid varies with location, so you have to apply a specific correction for your location (here in the Carolinas, it’s about 100 feet). You can do that with DD’s elevation calibration tool, but the problem is what Kaitlin describes above: since that calibration happens downstream of the processing, it won’t show on an accuracy report, and it’ll still look like you’re 100 feet off. BTW, @GregO, this is not an RTK issue - at least using the CORS stations in NC and SC as RTK correction sources - because those stations correct the GPS location for the variables of the GPS itself, not for the shape of the earth (ellipsoid-vs-geoid). So there are two potential solutions: 1) get a position correction that includes the ellipsoid-to-geoid correction; or 2) DD’s accuracy report moves downstream so that it captures the elevation calibration. A PPK source (rather than RTK) like EZ-Surv by Effigis provides solution 1. Do any of you on this thread know of a way to get that solution via RTK?
BTW, I’d like to thank both @Kaitlin and @Adam for their insights and help on this issue.