Digital Altimeter PRO is a new generation of altimeter for iphone / ipod touch.
The ground altitude is retrieved from the internet with your position.
This method will give you the best ground altitude precision.
Position calculation method:
iphone 3G and 3Gs: GPS, Cellular triangulation, WIFI.
Iphone V1: Cellular Triangulation, WIFI.
Ipod Touch: WIFI.
A internet connection in needed.