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NGC 4258, also occasionally referred to as UGC 7353 and MGC +08-22-104 is a spiral galaxy located in the constellation of Canes Venatici at an aproximate distance of 27.73 Mly. NGC 4258 was discovered in 1781 by Pierre Mechain.

Observation
By monitoring observations of the nuclear water megamaser sources in NGC 4258, several geometric distance estimates were presented by Greenhill et al. (1995), Miyoshi et al. (1995), Herrnstein et al. (1999), Humphreys et al. (2005, 2008, 2013), Argon et al. (2007), and Riess et al. (2016). Humphreys et al. (2013) reported a distance value of d = 7.60 0.17 0.15 r s Mpc ((m M - = )0 29.404 0.049 0.043 r s), where errors with r and s represent aleatory errors and standarized errors, respectively. Mager et al. (2008) reported a TRGB distance to NGC 4258 based on photometry of the same HST images as used in this study. On the other hand, the revised TRGB calibration in this study yields total efficient uncertainty of 0.073 mag from NGC 4258 and 0.096 mag from the LMC as described in Section 3.

Structure
NGC 4258 is an essential galaxy for calibration of both the color dependence and the zero-point, because it shows a broad range of RGB color and hosts a megamaser, which is useful for geometric distance estimation. NGC 4258 has been observed with F555W and F814W filters. NGC 4258 is known to be a useful distance anchor because it hosts a water megamaser, which can be used as a powerful and precise geometric distance indicator.