Cheeseman's factor for ground effect-based on image sources-has widely been used to account for the increase in thrust experienced by a lifting rotor in ground effect. However, for elevations lesser than half the rotor radius, the factor is known to drastically over predict this thrust augmentation. Even though these elevations may not be viable to full scale rotorcraft due to the fuselage height, they are quite commonly encountered in UAVs and MAVs. A new factor is proposed based on image vortices that is able to predict the thrust augmentation more accurately. Results are compared with multiple experiments from literature and are found to match exceptionally well. © 2017 by the American Helicopter Society International, Inc. All rights reserved.