One of the two main coordinate systems used by the
survey, with coordinates eta and lambda.
This is simply another spherical coordinate system, where (eta,lambda)=(0,90.) corresponds to (ra,dec)=(275.,0.) and (eta,lambda)=(57.5,0.) corresponds to
(ra,dec)=(0.,90.). Note also that at (eta, lambda)=(0.,0.),
(ra,dec)=(185.,32.5). So, this is a pure rotation of the usual RA/Dec system, as opposed to the great circle system, which is defined relative to each individual stripe.
For some reason, although "eta" is constant along great
circles, it is referred to as "survey latitude," while "lambda" is referred to
as "survey longitude." Also, "eta" runs only from -90. to 90.; the back of
the sphere is covered by "lambda", which runs from -180. to 180. The Survey
coordinates are defined such that the "primary" area of a stripe (otherwise known as a stave) in the north
is defined by a rectangle in Survey coordinates which is 2.5 degrees wide in
eta (coordinate width). You may be interested in SM routines which convert
between Equatorial, Survey, and Great Circle coordinates.
Again, Astrotools has routines to do this as well.