Altimeter app is very useful and accurate!

I use this app regularly while back country skiing in the Wasatch Mtns of Utah, and also climbing in the Tetons. It is very accurate when compared to known summit altitudes, and is an excellent navigational aid. Paired with the aneroid altimeter on my watch it is sometimes helpful in weather prediction, too.
Recent upgrade is excellent. Gives faster info; seems to use less battery power.

Math logic about Altimeter+