Large aperture thin film optics typically have substantial shape errors which are difficult to quantify with conventional measurement methods, such as interferometry. The interferograms are normally too complex and contain too much information to discern relevant shape information pertinent to (for example) solar energy collection. This has led to the development of the laboratory-oriented, less expensive and less complex measurement procedure presented here. The method is based on well-defined geometric relationships and provides a novel approach to measuring the shape of large optics that are known to have substantial surface distortion. Once the local reflection (surface tilt) angles are determined, the overall shape can be reconstructed and further analyzed using ray tracing software to obtain aberration coefficients if desired. This technique lends itself to being easily scalable to very large membrane structures and the measurement precision is only limited by the equipment chosen.