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 .