I tend to buy from amazon.com, gpscity.com, and tigergps.com , but I've also purchased a GPS and cables from an ebay.com seller with good success.
The altimeter is self-calibrating, but to be more accurate you can manually calibrate it by entering a known altitude/pressure. If you go to garmin.com you can download manuals for all their GPS units and read the details.
The altimeter is self-calibrating, but to be more accurate you can manually calibrate it by entering a known altitude/pressure. If you go to garmin.com you can download manuals for all their GPS units and read the details.