Try this
Private Function CalcAz(lat1 As Double, lon1 As Double, lat2 As Double, lon2 As Double, d As Double) As Double
Dim X As Double
Dim phi As Double
X = Cos((Sin(lat2) - Sin(lat1) * Cos(d)) / (Sin(d) * Cos(lat1)))
If Sin(lon2 - lon1) < 0 Then phi = X
If Sin(lon2 - lon1) > 0 Then phi = 2...