Otterpilot

What your trying to do is calculate "True Altitude" If you set your altimeter to zero on the ground then you have already corrected for pressure and the only thing you need to do is correct for temperature. Otherwise you need to take your indicated altitude from when you jumped and correct it first for pressure. Use the pressure reading on the ground and adjust it against standard pressure (29.92mb) If you were at 10,000ft and surface pressure was 30.32 you had a pressure altitude of 9600ft. You would then require the surface temperature and the temperature at altitude to convert this to True Altitude. I forget the calculation but if the temperatures are below standard than you will be lower than your pressure altitude. Standard temperatures are 15 degrees Celsius at sea level and drop 2 degrees per thousand feet. Generally speaking, skydiving on hot days with warm upper temps will mean you are jumping from a higher altitude even ithough your altimeter will not say so. Pressure levels are closer together when its cold so you would actually be lower when it's unusually cold.