Altitude Plus is a GPS based altimeter.

Features:
Current altitude. (Height from sea level)
Vertical accuracy.
Position. (latitude, longitude)
City name. (Requires internet connection)
It also allows you to save favorite places information (Place name, altitude, location, picture, shows location in map)
New widget
Share on Twitter, Facebook, Email, Text.
Save data to iCloud and keep data synchronized between devices.

You can configure to display altitude in meters (metric system) or feet.
It also allows you select the desired vertical accuracy.