This is the method used to calculate pixel coordinates in the Polar VIS CDFs.

In the image coordinate system, the X axis is the center line-of-sight in the
look direction; the Y axis is the cross product of the spin axis and the X axis; 
and the Z axis is the cross product of the X axis and the Y axis.  

When the display orientation conventions in the variable 3 description are applied, 
the low resolution camera image is rotated so that both Earth camera and low 
resolution camera images are displayed with Y axis pointing up and Z axis pointing 
toward the right.  

To obtain the coordinates of the observed position of a pixel, calculate the 
intersection of the line-of-sight with the surface of an oblately spheroidal Earth 
at the altitude given as variable 7.  The equation of the spheroid is 
     X**2/(A+ALT)**2 + Y**2/(A+ALT)**2 + Z**2/(B+ALT)**2 = 1 
where A is the Earth radius at the equator, B is the Earth radius at the pole, and 
ALT is the given altitude.  The line-of-sight equations are 
     (X-SCX)/DX = (Y-SCY)/DY = (Z-SCZ)/DZ 
where (SCX,SCY,SCZ) is the spacecraft position vector GCI, and (DX,DY,DZ) is the 
look direction unit vector GCI.  Solve the line-of-sight equations for two variables 
in term of the third; substitute into the spheroid equation; and use the quadratic 
formula to solve for the third variable.  Select the solution point closer to the 
spacecraft.