when map skills are critical

The gemara (Brachos 58) quotes Shmuel as saying that he is as familiar with the paths of the stars in heaven as he is with the streets of Naharda’ah [his home town].  The gemara could have told us simply that Shmuel was an expert astronomer – why did Shmuel use this particular expression to convey that point?  The Nishmas Avraham (p 376, intro to siman 328, 6:1) quotes R’ Shlomo Zalman as explaing that the gemara is not juat using an empty cliche, like we might say "knowing something like the back of your hand." Rather, the gemara' expression is meant to be taken literally and tell us something important agav uracha: Shmuel took the time to become an expert in every street and byway in Naharada’ah.  The reason Shmuel did so is because he was a doctor (B.M. 107) and might be called on to deal with pikuach nefesh situations.  With no GPS system to rely on, Shmuel had to know the fastest way to get to a patient when minutes could mean the difference between life and death.