As you can see from the example of Mt. San Gorgonio observed at a distance of 87 miles, if you ignored the curvature of the Earth you would calculate an elevation angle that would be too large by nearly a factor of two. It is easy in Southern California to observe peaks at such distances. So instead of looking up to see San Jacinto from Mt. Wilson, one actually looks exactly along a level line. This page gives the formulae needed to calculate the observed elevation angles of a peak observed from another location, accounting for the curvature of the Earth and atmospheric refraction. The calculated parameters of typical peaks that can be seen from a number of the peaks in the SGM are given in a separate table .
Atmospheric refraction makes the elevation angles of distant peaks slightly larger.
Atmospheric refraction is caused by the varying density of air with height. I also give the formulae for distances and azimuths. The formulae will assume that north latitudes and west longitudes are of positive sign, with south latitudes and east longitudes of negative sign. The cosine function for an angle x. The sine function for an angle x.