  Projection error limits the use of vector magnetograms of active regions
(ARs) far from disk center. In this Letter, for ARs observed up to 60o from
disk center, we demonstrate a method of measuring and reducing the projection
error in the magnitude of any whole-AR parameter derived from a vector
magnetogram that has been deprojected to disk center. The method assumes that
the center-to-limb curve of the average of the parameter's absolute values
measured from the disk passage of a large number of ARs and normalized to each
AR's absolute value of the parameter at central meridian, gives the average
fractional projection error at each radial distance from disk center. To
demonstrate the method, we use a large set of large-flux ARs and apply the
method to a whole-AR parameter that is among the simplest to measure: whole-AR
magnetic flux. We measure 30,845 SDO/HMI vector magnetograms covering the disk
passage of 272 large-flux ARs, each having whole-AR flux >1022 Mx. We obtain
the center-to-limb radial-distance run of the average projection error in
measured whole-AR flux from a Chebyshev fit to the radial-distance plot of the
30,845 normalized measured values. The average projection error in the measured
whole-AR flux of an AR at a given radial distance is removed by multiplying the
measured flux by the correction factor given by the fit. The correction is
important for both the study of evolution of ARs and for improving the accuracy
of forecasting an AR's major flare/CME productivity.
