  Sunspot area data play an important role in the studies of solar activity and
its long-term variations. In order to reveal real long-term solar variations
precise homogeneous sunspot area databases should be used. However, the
measured areas may be burdened with systematic deviations, which may vary in
time. Thus, there is a need to investigate the long-term variation of sunspot
area datasets and to determine the time-dependent cross-calibration factors. In
this study, we investigate the time-dependent differences between the available
long-term sunspot databases. Using the results, we estimate the correction
factor to calibrate the corrected daily sunspot areas of Debrecen
Photoheliographic Data (DPD) to the same data of Greenwich Photoheliographic
Results (GPR) by using the overlapping Kislovodsk and Pulkovo data. We give the
correction factor as GPR=1.08(\pm 0.11)*DPD
