Ronchi interferometry is an optical testing technique similar to shearing interferometry. A coherent wavefront is interfered with a sheared form of itself by placing a periodic grating at, or near, the focus of an optical system. The resultant interference pattern contains information about the wavefront's slope in a direction perpendicular to the grating structure. The wavefront can be reconstructed from two orthogonal slope data sets via the process of sampling, ordering, and fitting. This paper develops a linear-algebra vector notation model of the interferogram sampling and fitting process.