OK here is what I learned from my brother.
You want to use the Geoid values, however you also want to make sure you have the Geoid model loaded in to your Trimble R8 GPS. (for those not paying attention that is a survey grade instrument with cm accuracy). The geoid numbers are tied to the map projection you are using and they are also tied to the GPS ground stations you are connected to 9necessary for the cm grade accuracy). The Ellipsoid values assume that the earth is perfectly/mathematically round. Geoid values correct for the fact that it is not perfectly round and the reason why your GPS needs the local Geoid data set loaded before you take readings. You do have the Geoid data for your area loaded don't you? For radio mobile you will want to use the Geoid height converted to meters. Is there LIDAR data for the area in question? It might be easier just to use a value from that. Might also be nice to convert the LIDAR to a file format for Radio Mobile and use all that data (going to need a lot of disk space though). Thank you, Brian Webster www.wirelessmapping.com From: AF [mailto:af-boun...@af.afmug.com] On Behalf Of ch...@wbmfg.com Sent: Tuesday, May 5, 2020 2:12 PM To: af@af.afmug.com Subject: [AFMUG] Elevation question I have a pretty good GPS receiver. Trimble R8 with a Yuma data collector. Normally we use it only for lats and longs. It is corrected in real time with a cellular data modem and a RTK/VRS ground station network. Yesterday we did a shot from a hill that did not exist when shuttle radar data was taken. It was way off in elevation. So we took a shot from our parking lot and compared it with what we think we know to be the altitude as well as our cell phones and google earth. Turns out the R8 has an ellipsoid height as well as another height associated with the northing and easting values. The second one seems to be more in line with what I was expecting. The local dealer sent me a drawing showing this. (attached) So now I am super confused. Not sure what number to use when working with Radio Mobile. Do I add the geoid height to the ellipsoid height? Was hoping Brian Webster may know or others may know. Googling google earth elevation talks about different versions of geoid. Not sure if their elevations are geoid elevations or not. So, rather than dig into it any more I am taking the lazy approach and posting the question here.
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