Thread: GPS and USGS topos disagree

I have a Garmin GPSMap 76CSx that I've had for quite a while now.
I have noticed on occasion that the GPS's altitude function seemed to be off in its own little world.
I've used it quite a bit recently around my property to locate and mark things.

I consistently get altitude reading off the GPS of 805' to 815' of altitude around my entire lot.
I have been downloading the USGS' newer 2013 DRG GEO PDFs of the topo maps in my vicinity.
These DRG GEO PDFs have an orthoimage satellite photo layer.
These images locate my house with the 760' elevation line running right through it, about 45' to 55' lower than the GPS says it is.

The elevation contours are the same as in the older editions of this map.

So my question:
What's going on?

EDIT:

as my dear old mother used to tell me:
"Open your eyed and shut your mouth!"

After posting the preceding, I went looking for the GPS' manual.
I found the "Calibrate Altimeter" section and calibrated it to 760'.
If that doesn't do the job, I'll be back.

I now remember, back when I first bought it, I almost always used it at sea level for kayak travel.
I didn't need an altimeter function.
So, I didn't even try calibrating it.
.