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I've never written a travelogue, but of the few I've read, it depends on your intention. As one of your American readers trying to retrace your steps or find specific landmarks you mention, I would...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/8398 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I've never written a travelogue, but of the few I've read, it depends on your intention. As one of your American readers trying to retrace your steps or find specific landmarks you mention, I would prefer US units -- unless I'm traveling by car with instruments in local units. If you are more interested in capturing the experience and sense of place, I would prefer local units. I would recommend against compromising with qualifiers, parentheticals, etc. Choose one and stick with it. If you have just a ton of usages, maybe include an appendix with a conversion chart. Ultimately, though, this is the sort of decision your publisher or editor gets final say on. So whatever you decide, if this is an unsolicited manuscript, it might not matter since your editor/publisher might ask you to change all the units anyway.