  We proposed a new method -- "Pacman" -- to calculate Faraday rotation measure
(RM) maps from multi-frequency polarisation angle data (Dolag et al.) in order
to avoid the so-called n-pi ambiguity. Here, we apply our "Pacman" algorithm to
two polarisation data sets of extended radio sources in the Abell 2255 and the
Hydra A cluster, and compare the RM maps obtained using "Pacman" to RM maps
obtained employing already existing methods. Thereby, we provide a new high
quality RM map of the Hydra north lobe which is in a good agreement with the
existing one but find significant differences in the case of the south lobe of
Hydra A. We demonstrate the reliability and the robustness of "Pacman". In
order to study the influence of map making artefacts, which are imprinted by
wrong solutions to the n-pi ambiguities, and of the error treatment of the
data, we calculated and compared magnetic field power spectra from various RM
maps. The power spectra were derived using the method recently proposed by
Ensslin & Vogt (2003). We demonstrate the sensitivity of statistical analysis
to artefacts and noise in the RM maps and thus, we demonstrate the importance
of an unambiguous determination of RM maps and an understanding of the nature
of the noise in the data. We introduce and perform statistical tests to
estimate the quality of the derived RM maps, which demonstrate the quality
improvements due to "Pacman".
