Satalite Navigation Unit

Got a Garmin e-trex Vista Hxc direct from ERSI in BKK. They threw in the Thailand map for free. Good unit but it doesn't talk as it's a hand held portable unit more designed for trecking than driving. The altitude measurement is barometric, but unless you know the height and pressure when you calibrate it the measurement will be out. At least over a short period of time it will accurately tell you the difference between a high and low point. Over several hours, when the barometric pressure at the location has changed the reading will become more and more inaccurate until recalibrated.

Yep, that's how barometric altimeters work. They should be reset daily or more if weather changes.

Easy enough to reset them here as sea level is nearby.

I wouldn't trust them to be more accurate than perhaps 20 meters. (depending on when you set it last at a known point)