  In order to resolve and characterize anisotropy in turbulent plasma flows a
proper estimation of the background magnetic field is crucially important.
Various approaches to calculate the background magnetic fields, ranging from
local fields to globally averaged fields, are commonly used in the analysis of
turbulent data. Here we investigate how the uncertainty in the orientation of a
scale dependent background magnetic field influences the ability to resolve
anisotropy. Therefore we introduce a quantitative measure, the angle
uncertainty, which characterizes the uncertainty of the orientation of the
background magnetic field which turbulent structures are exposed to. The angle
uncertainty can be used as a condition to estimate the ability to resolve
anisotropy with certain accuracy. We apply our description to resolve spectral
anisotropy in fast solar wind data. We show that if the angle uncertainty grows
too large, the power of the turbulent fluctuations is attributed to false local
magnetic field angles, which may lead to an incorrect estimation of spectral
indices. In our results an apparent robustness of the spectral anisotropy to
false local magnetic field angles is observed, which can be explained by a
stronger increase of power for lower frequencies when the scale of the local
magnetic field is increased. The frequency dependent angle uncertainty is a
measure which can be applied to any turbulent system.
