There are 4 altitude references and 1 pressure reference that SoarPilot can display and use. These are explained on the diagram below.

• MSL - Altitude above Mean Sea Level
• AGL - Height above the ground (requires a Terrain database to be loaded)
• FL - Flight level altitude (assumes sea level pressure is 1013.2mb or 29.92 in Hg)
• QFE - Height above an specific reference. See the QFE/QNH Settings Screen.

• QNH - The local air pressure at sea level. See QFE/QNH Settings Screen.

This most important of these is the MSL setting. This is the height of a point above the Mean Sea Level. All the elevations of waypoints should be a MSL height. All SoarPilot final glide calculations are based on MSL height.

Also important for airspace display and warnings is FL or flight level. This is the altitude shown on an altimeter with the sub-scale set at 1013.2mb or 29.92 in Hg. Many airspace areas have FL altitudes, and the pilot needs to be aware that the actual MSL height of these areas will change with the local air pressure at the time of the flight.

At first this may seem strange, but if you are flying long distances across weather systems, the local air pressure on the ground will change, so a constant reference is required to ensure that everyone has the same understanding for the altitude.

If the local pressure is high, the flight levels will be higher. So FL45 will be greater than 4500ft MSL. Conversely if the pressure is low, the flight levels will be lower.

Flight levels are always in feet.