Simplified methods of the magnetic diagnostics of tokamak equilibria are revisited. These methods allow interpretation of external magnetic measurements in the form of simple analytical formulas for physical parameters, as linear functions of measurements, similar to the ones used for high aspect ratio circular plasmas. At present, these methods can be considered as simplified versions of general ones, such as toroidal harmonics, local field expansion and function parametrization methods. A simplified function parametrization method is developed for simple non-circular plasma shapes. An error analysis of the various methods is performed by numerical simulations for the TCA/BR tokamak with moderate aspect ratio and plasma elongation up to 1.4.