I think its somewhat akin to a tiltmeter - its very local in its method.

I suppose this would work with any flat surface that is north-south as long as you have a good clearance on the east or west side for the sun to shine on such as the side of a house... Or an existing wall... All you really need is one side. East if you mark in the mornings and west if in the afternoons. Is that right?

Canie