Most of the standard regions use conformal (equal-area) projections.
For such projections, the area of each pixel is constant and equation
to the nominal pixel dimension (4.45 km for eggs, 2.225 km for slices)
squared.  However, polar regions use a polar stereographic projection.
This projection does not have constant area pixels.  For these regions,
SIR-format images that contain the pixel area in square km for each
pixel are provided here.