A spatial calibration for conditional frequency analysis, the inversion technique applied to routine back trajectories to study the association between geographical regions and conditions at a receptor, is discussed for trajectories arriving at Hopi Point, Arizona. Several 0.5¿¿0.5¿ cells were selected at three different distances from Hopi Point. These cells were used to mimic sources, with air residing over them considered contaminated by imaginary emissions. The distribution of back trajectory segment endpoints over a latitude-longitude grid was then examined as a function of the trajectories sojourn over each imaginary source cell on their way to the receptor. The areal extent of significant associations, evaluated for different seasons and the various ideal source positions, suggests that this inversion technique can resolve the direction of potential sources but has limited resolution for their radial distance from the receptor. ¿ American Geophysical Union 1996