Setting the takeoff altitude correctly could be the difference between a successful and a frustrating end of the flight.
The altitude in the Oudie 3 is recorded at the standard pressure of 1013.25 hPa. In order to know you current altitude above mean sea level and your current flight level altitude the altitudes need to be corrected to correspond with the real world.
There are several ways how the Oudie 3 can correct the takeoff altitude. It will typically assume that the pressure recorded by the pressure sensor is relative to the standard atmosphere pressure of 1013.25 hPa (but it is even possible to correct for that, see below). It is quite possible to correct the altitudes to QNH automatically with the data which is already available on the Oudie 3. If you get the information from an external source (such as the ATC tower or the competition task sheet) then it is possible to enter the values manually as well.
In order to correct the takeoff altitude you need to go to Menu > Vol & Alt. dialog.
By default the Auto QNH checkbox is checked. The Auto QNH feature is going to average the GPS Altitude for a very long time to eliminate random errors from the GPS Altitude. Then it will correct the pressure altitude to the measured GPS Altitude by changing the QNH Pressure setting. That gives you a useful takeoff altitude suggestion as well as an indication of the QNH pressure level. Note: In order for this method to work well you need to turn on the Oudie for at least 5 minutes, 10 is better. Once Auto QNH has been determined you can put the Oudie back to sleep. It will remember the measured values and start from there once you turn it back on.