Deuterium-tritium (D-T) single-crystal ice layers in spherical shells often form with localized defects that we believe are vapor-etched grain boundary grooves built from dislocations and accommodating slight misorientations between contacting lattice regions. Ignition implosion target requirements limit the cross-sectional areas and total lengths of these grooves, and since they are often the dominant factor in determining layer surface quality, it is important that we be able to characterize their depths, widths, and lengths. We present a variety of ray-tracing and diffraction image modeling results that support our understanding of the profiles of the grooves, which is grounded in X-ray and optical imaging data. We also describe why these data are nevertheless insufficient to adequately determine whether or not a particular layer meets the groove requirements for ignition. We present accumulated data showing the distribution of groove depths, widths, and lengths from a number of layers, and we discuss how these data motivate the adoption of layer rejection criteria in order to ensure that layers that pass these criteria will almost certainly meet the groove requirements. We also describe future improvements that will provide more quantitative information about grooves in D-T ice layers.