Measuring the range size of a species is essential if we are to assess its current status and thereafter analyse changes in its status. The usual approach is to examine grid-square distribution maps, and count up the numbers of squares in which the species occurs. However, each 2km (tetrad) or 10km (hectad) grid square contains land that is not occupied by the species in question, leading to systematic biases in estimates of current status and rates of decline.
