Comparison of geometrical mapping for ring diagram analysis

Mapping the solar surface is a crucial step in any local helioseismology technique. Because the acoustic waves propagate along great circles at the solar surface, it has been shown that these circles need to be used in the geometrical construction of the plane grid. We study different types of projections based on great circles for the calculation of sub-surface flows from ring diagram analysis of GONG data. Azimuthal equidistant projection, transverse cylindrical projection, gnomonic projection and stereographic projection produce almost the same velocity fields with standard patch sizes (15 degrees x 15 degrees). The difference between the four projections is more noticeable when larger patches (30 degrees x 30 degrees) are used.