I have used various GPS units- CF card, Stand alone, Pocket PC coupled with or without blue tooth, here in the US and elsewhere in the world; GPS derived elevation figures are not as accurate or reliable as Barometric readings or digital elevation maps (DEM) derived from satellite imagery/altimetry and LIDAR. Apparently, direct elevation readings for most locations will be embarrassingly off. This is caused, in part, because the GPS definition of altitude does not refer to Mean Sea Level (MSL), but rather to a gravitational surface called the reference ellipsoid. Another contributing factor is the vertical distance triangulation from differences in timing of received radio waves at the antenna of the unit which are distorted due to ground reflections and atmospheric scattering.
To answer your question Re: elevation reading on the NAV unit, I cant recollect offhand but it may be available under system settings or some such on the NAV menu..