Fer de Vries presents a mathematical method for calculating sundial lines and for determining the inclination and declination of a dial plane. He defines various coordinate transformations needed to convert the sun's position into shadow-point coordinates on any surface, applicable globally. The procedure can also be reversed to find time and date from a shadow, or to determine the dial's orientation from observed shadow points, and is useful for designing mirror or submersible dials.
Mathematics of Dialling,
Sundial Design & Layout