Some authors have determined patch perimeters in raster image processing and geographic information systems by summing the number of pixels immediately surrounding the patches. We demonstrate that this method is inaccurate. When applied to satellite sensor imagery, errors ranged from overestimation of perimeter length by 34-5 per cent to underestimation by 41·7 per cent. Therefore. we developed a new method that is both accurate and relatively simple. This method determines perimeters using a standard 3 by 3 pixel moving window. The method can be used with most raster systems and has applications in landscape ecology for calculating such variables as the fractal dimension and ecotone dimensions