To identify whether a single-phase grounding fault occurs in a cable section or an overhead line section based on differences in transition resistance, this paper investigates estimation methods for transition resistance in small-current grounded distribution networks. The fundamental principles of four correction methods are discussed, including total approximation (TA) based on the detected zero-sequence voltage, zero-sequence current, and phase voltage; grounding current amplitude correction (GCAC); grounding current vector correction (GCVC); and GCVC plus equivalent zero voltage source correction (GCVC + EZVC). Next, a quantitative error analysis of these methods is conducted, along with a discussion of the impact of typical load distribution. The study shows that while TA has relatively large errors, all the correction methods significantly improve the estimation accuracy of the transition resistance. With GCVC+EZVC, in the case of typical feeder load distribution, although the relative errors are large during low-resistance grounding, the absolute error remains small enough to meet practical requirements. Finally, simulation and experimental results verify the feasibility and effectiveness of the proposed method.