Originally Posted by jmden
GPS is the best way.
With fluctuating barometric pressures, altimeters can lead you astray easily. Ballistically, the best way to calculate air density is to use a thermometer and a pressure guage. Using raw pressure you can completely forget about altitude and set the program to zero feet.
If you cannot afford a guage, then altitude is your next best option and as jmden stated, a GPS is by far the most accurate followed by a good topo map.
The Brunton units are pretty economical versus the Kestrals.