Having issues with elevation

I am new to Reach and just took delivery of a Reach RS+. I was able to successfully configure everything and I also setup NTRIP using RTK2go.com. Everything seems to be working fine. Set the reach on a known point that was surveyed and got excellent results, except for the ASL. It’s way off. I know I am doing something wrong but cannot figure it out. One survey map states that the area the reach is in is 30.48m. The reach say’s it’s 18.66m with a 0.010 error. A cheap old junky GPS say’s it’s 22m so figuring with the error possibilities it’s confirming the reach. My Iphone Altitude app say’s its 48m. The DJI Matrice 30T says it’s 42m. If I remove the corrections the ASL doesn’t change much, it bounces around in the high teens but the error is +/- 7 meters, which still doesn’t account for the huge variation. And lastly, this is actually in my back yard and I have always known our altitude to be over 31m. Of course the next step is to go out and find a monument someplace but I am wondering if I am not interpreting the data correctly or have something misconfigured.

This will likely help you understand what you are seeing:
Geoid - Wikipedia.

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Yes it explains it all, thank you.

Hi George,

Just thought to ask if you were able to get the necessary height with your Reach RS+. Do you need any help with it?