In GILDAS, the effect of the Earth curvature is approximated by a correction to the usual airmass definition (See Eq. 5). Fig. 2 displays the airmass as a function of the elevation for 3 different atmosphere models: 1) a plane parallel atmosphere, 2) a curved atmosphere with a scale height of 2.7 km, and 3) a curved atmosphere with a scale height of 5.5 km. It is clear that as long as the elevation is larger than 20 deg, the difference in airmass between these different atmosphere models is negligeable.
