地球の裏の緯度経度を求める

Public Function GetBackLatLon(FromPoint As IPoint) As IPoint
    '入力座標が地理座標系でない場合、Nothingを返す
    If FromPoint.SpatialReference Is Nothing Then
        Set GetBackLatLon = Nothing
        Exit Function
    ElseIf Not TypeOf FromPoint.SpatialReference Is IGeographicCoordinateSystem Then
        Set GetBackLatLon = Nothing
        Exit Function
    End If
     
    Dim pPoint As IPoint
    Set pPoint = New Point
    Set pPoint.SpatialReference = FromPoint.SpatialReference
     
    '経度の計算
    If FromPoint.X > 0 Then
        pPoint.X = FromPoint.X - 180
    Else
        pPoint.X = FromPoint.X + 180
    End If
     
    '緯度の計算
    If FromPoint.Y > 0 Then
        pPoint.Y = FromPoint.Y - Abs(FromPoint.Y) * 2
    Else
        pPoint.Y = FromPoint.Y + Abs(FromPoint.Y) * 2
    End If
     
    Set GetBackLatLon = pPoint
 
End Function