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The interference between two radar images, acquired from approximately the same location, has a number of uses. In particular, the literature abounds with examples of coherent change detection (CCD) and interferometric synthetic aperture radar (InSAR) uses that, respectively, enable the detection of small changes and the extraction of terrain elevation. In almost all of these examples, collections have been deliberately constrained so that they have almost identical collection geometries. Details of the variation in geometry have been analyzed extensively but generally only with respect to variation in the mean grazing angle. Much of this variation is due to the angular variation in the scene reflectivity; however, when collection geometries differ in other respects, the reduction in performance cannot be entirely explained by angular variations alone. This paper identifies a source of performance degradation when the collection geometries are different for scenes containing local relief. The error is due to approximations during image formation that both the collection geometry and the terrain are flat. The effect of these errors is derived as a blurring kernel to the true scene. The impact of the blurring is then assessed in the interferometric case and broken into common and differential blurring terms to facilitate the impact of some special cases and to explain why the parallel-pass geometry works as well as it does even though the true three-dimensional nature of the collection and the ground are ignored.