For map distance unit, append unit d for arc degree, m for arc minute, and s for arc second, or e for meter [Default unless stated otherwise], f for foot, k for km, M for statute mile, n for nautical mile, and u for US survey foot. By default we compute such distances using a spherical approximation with great circles (spherical=:greatcircle) using the authalic radius (see PROJ_MEAN_RADIUS). You can use spherical=:flat to perform “Flat Earth” calculations (quicker but less accurate) or spherical=:ellipsoidal to perform exact geodesic calculations (slower but more accurate; see PROJ_GEODESIC for method used).