Limitations.--All shape indices based on perimeter-area relationships have important limitations. First, perimeter lengths are biased upward in raster images because of the stair-stepping pattern of line segments, and the magnitude of this bias varies in relation to the grain or resolution of the image. Thus, the computed perimeter-area ratio will be somewhat higher than it actually is in the real-world. Second, as an index of "shape", the perimeter-to-area ratio method is relatively insensitive to differences in patch morphology. Thus, although patches may possess very different shapes, they may have identical areas and perimeters. For this reason, shape indices based on perimeter-area ratios are not useful as measures of patch morphology; they are best considered as measures of overall shape complexity. Alternative indices of shape that are not based on perimeter-area rations are less troubled by these limitations. But these too, generally do not distinguish patch morphology, but instead emphasize one or more aspects of shape complexity (e.g., elongation).