We use photoemission electron microscopy to measure the ferroelastic twin wall angles at the surface of CaTiO_{3} (001) and deduce the strain ordering. We analyze the angular dependence of the photoelectron emission from different domain surfaces, each with its own characteristic tilt angle in the factory rooflike topography. By considering the surface topography as a field perturbation, the offset in the photoemission threshold can be directly related to the tilt angles. With knowledge of the symmetry allowed twin walls we quantify the twin angles between 179.1° to 180.8°.