The India-based Neutrino Observatory (INO) collaboration, a multi-institutional effort to build a 50 kton magnetized Iron Calorimeter (ICAL) for studying neutrino oscillations. The magnetic field in the ICAL, induced by copper coils, will be a crucial input for the track fitting algorithms that reconstruct the four-momenta of atmospheric neutrinos. The magnetic field map as estimated from finite element techniques is usually not accurate owing to many factors including approximations in the simulation and the change in the elemental composition of the iron plates, etc. In this work, we present the results of a simulation study that employs the muon spin-rotation technique to estimate the magnetic field.