A subdivision of a strip. Each strip is covered by 6 "scanlines". Scanlines are defined in
great circle coordinates. The great circle coordinate
system is different for each strip, and is defined by setting the
equator of the coordinate system to be the center line of constant eta
for the stripe to which the strip belongs. A scanline is then bounded on the top and bottom by
lines of constant nu, with no east or west boundaries.
Scanlines touch but don't overlap, and thus are a unique mapping on the
sky for that stripe only. Scanlines for different stripes DO overlap. This is because the scanlines come from the camera columns, or camcols, which
have a fixed physical width, while the spherical coordinates converge at the poles. Note: At times, the term scanline has been used interchangeably (and improperly) with camcol.