  The CMS detector is designed around a large 4 T superconducting solenoid,
enclosed in a 12000-tonne steel return yoke. A detailed map of the magnetic
field is required for the accurate simulation and reconstruction of physics
events in the CMS detector, not only in the inner tracking region inside the
solenoid but also in the large and complex structure of the steel yoke, which
is instrumented with muon chambers. Using a large sample of cosmic muon events
collected by CMS in 2008, the field in the steel of the barrel yoke has been
determined with a precision of 3 to 8% depending on the location.
