What is the relationship between surface azimuth and surface-solar azimuth in determining the angle of incidence?

The surface azimuth (ψ) is the direction of the surface normal relative to true south, while the surface-solar azimuth (γ) is the direction of the sun relative to true south. To determine the angle of incidence (θ) between a direct solar beam and the normal to the surface, both the surface azimuth and surface-solar azimuth must be known. The angle of incidence can be calculated using the equation: θ = arcsin(sin(ψ) \* sin(γ) + cos(ψ) \* cos(γ) \* cos(β)), where β is the solar declination angle.