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Rovibronic energy levels are determined for four low-lying electronic states (\X, \A, \Sa, and \Sc) of the imidogen free radical ($^{14}$NH) using the \Marvel\ (Measured Active Rotational-Vibrational Energy Levels) technique. Compilation of transitions from both laboratory measurements and solar spectra, found in 18 publications, yields a dataset of 3002 rovibronic transitions forming elements of a measured spectroscopic network (SN). At the end of the MARVEL procedure, the majority of the transitions form a single, self-consistent SN component of 2954 rovibronic transitions and 1058 energy levels, \NoX, \NoA, and \Noc\ for the \X, \A, and \Sc\ electronic states, respectively. The \Sa\ electronic state is characterized by \Noa\ $\Lambda$-doublet levels, counting each level only once. Electronic structure computations show that unusually the CCSD(T) method does not accurately predict the \Sa\ excitation energy even at the complete basis set limit.