Controlling thin-film stress magnitudes and nonuniformities is a persistent and pervasive challenge for applications ranging from the semiconductor industry to nanotechnology. Consequently, the ability to accurately measure the intensity and spatial distribution of film stress is essential for fabrication process optimization. This paper describes novel extensions of the membrane resonance method to characterize in-plane film stress gradients. Arrays of freestanding membrane windows are fabricated by first depositing thin films over a silicon substrate. Back-etching then creates an array of membrane windows which are sequentially resonated with a compatible drive electrode. The technique was used to determine stress uniformity across electron projection lithography masks, which incorporate membrane window arrays. In addition, a procedure is presented for identifying the two principal stresses in individual membranes.