  We present here detailed derivations of mathematical expressions for the
accuracy in the arrival direction of particles estimated using a set of stacked
resistive plate chambers (RPCs). The expressions are validated against
experimental results using data collected from the prototype detectors (without
magnet) of the upcoming India-based Neutrino Observatory (INO). We also present
a theoretical estimate of angular resolution of such a setup. In principle,
these expressions can be used for any other detector with an architecture
similar to that of RPCs.
