In Ding et al. [A unified approach for circularity and spatial straightness evaluation using semi-definite programming, International Journal of Machine Tools & Manufacture 47(10) (2007) 1646–1650], the authors advocate semidefinite programming-based relaxations of quadratic optimization problems as a vehicle to solve two circularity and straightness evaluation problems. The purpose of this comment is to point out that the use of semidefinite relaxations for the problems at hand are redundant, since the problems are convex, or changed inter-alia to convex problems. We also take the opportunity to clarify some properties of the semidefinite relaxation, were it to be used for an actual nonconvex problem in this area.