Vertical datum issues on iPhone

I use Gaia GPS on my iPhone. It works quite well. It reports the elevation in my backyard as 320 or so feet. It shifts slightly over time, but it's consistently in that range.

When I power on my inReach Mini and it connects via Bluetooth the elevation changes suddenly by a couple of hundred feet. (If memory serves, to about 125 feet.) This happens consistently. When I power the inReach off, the elevation snaps back to the prior value.

From what I can tell, the inReach should be using WGS84, and that's the setting on the app as well. (The setting on the app should ultimately be irrelevant, because the location services are being provided via the OS.

FWIW, the USGS quad map agrees with the iPhone's in-built GPS, not what I'm getting from inReach. Any thoughts on how to resolve this mismatch? (Sure looks like a vertical datum mismatch to me.)

iPhone 10 has the latest updates, same for the Mini.

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twolpert over 5 years ago +1

I would be more inclined to think that this is simply the result of a large vertical position error. The Mini has no barometric sensor. So you are dependent entirely on GPS for both horizontal and vertical…

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0 twolpert over 5 years ago

I would be more inclined to think that this is simply the result of a large vertical position error. The Mini has no barometric sensor. So you are dependent entirely on GPS for both horizontal and vertical position. IIRC, vertical error is typically around twice the horizontal error. Even when tracking, the Mini does its best to maximize battery life. It doesn't acquire a fix any more often than absolutely necessary, and the quality of the fix may not be particularly good (not that you can tell from the Mini itself).

It's also possible that there is a large local difference between the ellipsoid and the geoid. I have no idea if the Mini has a geoid model at all. This is the kind of detail that no manufacturer ever publishes. So it's possible (even probable) that you are seeing the unadjusted vertical position relative to the ellipsoid.

You can try one of the other available datums such as NAD27 CONUS or WGS 72. It would be interesting to know the results. But I don't necessarily think they will be better.