EPSG guidance note #7.
The simplest of all polynomials is the general polynomial function. In order to avoid problems of numerical instability this type of polynomial should be used after reducing the input parameters, usually coordinate offsets U and V relative to a central evaluation point, to ‘manageable’ numbers, between –10 and +10 at most. U = XS - XS0 in defined units (which may not be those of the coordinate reference system), V = YS - YS0 Then (XT - XT0) = (XS - XS0) + dX (YT - YT0) = (YS - YS0) + dY or XT = XS - XS0 + XT0 + dX YT = YS - YS0 + YT0 + dY where XT , YT are coordinates in the target coordinate reference system, XS , YS are coordinates in the source coordinate reference system, XS0 , YS0 are coordinates of the evaluation point in the source coordinate reference system, XT0 , YT0 are coordinates of the evaluation point in the target coordinate reference system. and where dX = A0 + A1.U + A2.V + A3.U2 + A4.U.V + A5.V2 + A6.U3 + A7.U2.V + A8.U.V2 + A9.V3 + A10.U4 + A11.U3.V + A12.U2.V2 + A13.U.V3 + A14.V4 dY = B0 + B1.U + B2.V +B3.U2 +B4.U.V +B5.V2 + B6.U3 +B7.U2.V +B8.U.V2 +B9.V3 + B10.U4 + B11.U3.V + B12.U2.V2 + B13.U.V3 + B14.V4