A length artifact has a first side wall, a second side wall, and a base portion, the first side wall and the second side wall separated by an air gap at least four millimeters wide, the base portion being attached to a bottom of the first side wall and the second side wall, the base portion further having a first platform region that includes a first nest and a second platform region that includes a second nest, the first nest and the second nest configured to accept a spherical surface of a retroreflector target.