Altimeter+ (iOS) app for FREE
This Altimeter determines the height using a digital elevation model. ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) provides digital elevation data with an accuracy of ±10m.
Alternatively, you can also only use GPS for height determination. The GPS altitude is less accurate.
ASTER only delivers the topographical elevation. Example: When ballooning, ASTER does not provide the height of the balloon, but the elevation of the underlying ground.+