As elevation is increased the air density is decreased causing a loss in horsepower. This page shows the decrease in pitch needed as elevation increases.   

Up to 2500’ – same as sea level

2501’ to 3500’ – deduct 1” of pitch

3501’ to 4500’ – deduct 2” of pitch

4501’ to 5500’ – deduct 3” of pitch

5501’ & up – deduct 4” of pitch