Adjust elevation/altitude on jbl navigation system

Your car doesn’t have a barometric reference knob?

More seriously, I don’t know of a way to adjust the elevation readout on Toyota navigation systems. The only field calibration mentioned in the manuals is done after replacing the tires, since unworn tires with a larger circumference turn slightly fewer revolutions per kilometer, which affects dead-reckoning accuracy.

Elevation errors of the magnitude you mention could be a combination of GPS positioning error and the difference between the actual mean sea level at your location and the WGS 84 reference ellipsoid used by GPS. See these pages for more information:

“Understanding the accuracy of the GPS elevation reading,” Garmin Support Center.
“Mean Sea Level, GPS, and the Geoid,” ArcUser Online, July-September 2003.

 

This is within civilian GPS vertical accuracy, which is poorer than horizontal accuracy. And it should vary significantly as the overhead satellite constellation changes, and even more so when traveling in topography (e.g. mountains and canyons) that restricts the antenna's view of the satellites.

How to fix it? Get a military GPS system, which uses more signals and corrects for more measurement error mechanisms. The U.S. government may not allow you to do this. Or get an aftermarket system that reads all three satellite systems, and choose whichever is working best at any particular moment: GPS (USA), Glonass (Russia), and Gallileo (EU).

Is your Nav equipped with barometric compensation, as my handheld hiking unit is? (My 2012 Prius Nav doesn't show altitude at all.) If so, then you'll need it to adjust it according to today's barometer reading, maybe even this hour's barometer reading. Even barometric altitude varies by more than the 'error' you report.