The geometry of the sensor and the vegetation canopy have an effect upon the variance of the detected spectral response of a single cover type in an image. The procedure for calculating the size of the ground resolution element (GRE) for a Daedalus ATM scanner is outlined, and examples of changes in variance for four land-cover types are presented. Image variance was found to be inversely related to sensor view angle, owing to the increased size of the GRE, the increased overlap of adjacent elements, and the changing proportions of canopy components viewed by the sensor.