GPS Not giving correct elevation

I have two units. One is GeoMax Zenith 25Pro base/rover and other is Sokkia GRX2 base/rover.

I set up the Sokkia over a few days to collect static data for several hours at a time connected to the internet to CORS. I got the position and elevation reading within hundredths over the three days I took data after running it through OPUS and averaged it.

I set up base/rover and used the OPUS solution to set the H/V. All was good with the Sokkia except it is terrible to get a fix in any kind of overhead obstruction. It stayed on Float and would not fix.

So the next day I used the OPUS solution H/V data I got with the Sokkia, for my control point for my GEOMAX as it tends to be better at getting signals in somewhat wooded areas where the Sokkia wouldn't even try.

I tried for two evenings to get the GEOMAX to read the correct elevation. But it kept reading about 0.3' lower than the Sokkia even though I used the same control. Both read at the same point on the units. I ended up adding a default of 0.3' elevation adjustment to the GeoMax get them to read the same. I don't understand why the GEOMAX would not read the same as the Sokkia. I measured everything and set heights and did everything exactly the same on both units.

What did I do wrong? They should have read the same.

This is a personal job I'm working on, doing a topo of part of my property for a septic/leach field but I can't for the life of me figure out why I had .3' elevation difference between the two even though both were set on the same control point on different days. I have a footing I set them on and when I poured it I checked it with my total station and none of it was out more than a little over a hundredth so I should have been able to shoot pretty much anywhere on the footing for an elevation check and it worked with the Sokkia but I got the .3' difference with the GeoMax.

I couldn't get my internet signal to reach out as far as I needed to get the shots using CORS. So I figured I could use my base and set it at the control point and just start collecting data. But that wasn't the case.

.3' isn't a big deal for what I'm doing (2' contours), but it has cost me two days of head scratching because I hate not getting awesome results even when they are not technically needed.

This is the first time I used CORS with internet to do any type of surveying. I rarely use GPS because I don't trust it in the woods after several times of double checking the GPS with conventional traversing, I found when the results were good they were really good, but when they were off, they were way off (several feet) and you never knew until you checked each shot with a good ol' closed traverse.

I generally only use it for control points in open areas on either end of larger tracts as another cross-check?ÿalong with a closed traverse.

I want both units to read the same but for some reason, the only way I could do this was to do an elevation adjustment on each shot I took with the GEOMAX so it would?ÿmatch the Sokkia.

I have not tried connecting the GEOMAX to CORS over the internet. Maybe that would shed some light on things.

Any other ideas? I have to have something set wrong somewhere for me to get a consistent .3' elevation difference between the units. I'm already almost bald, so if someone could steer me in the right direction, maybe, just maybe, I can save the little bit of hair I have on my head.

?ÿ