- airport elevation is MSL otherwise very airport would be 0
- obstacle are shown in both MSL/(AGL)
- on chart airspace altitude always in MSL pilot have no idea of field elevation
- altimeter is calibrated in MSL (same ref point for altitude)
- stack number in airspace is always in MSL
- Class G and E are in AGL because it follows the terrain contour
- Class G start at the surface and goes up to 700 feet or 1200 feet AGL
