Up/Down have come to mean North/South in the way many people speak in English (US-centric? Not certain how other English speaking countries handle this). I think that's what GP post was referring to.
That it's actually referring to upriver/downriver in the case of Egypt is lost on those who aren't more closely familiar with the geography. It makes sense once explained, but it's non-obvious if you've become accustomed to up and down being synonymous with North and South.
Side note: There's a discussion about Common Core math and a comment by DanBC about how students may correctly fill out many math worksheets without developing any actual understanding just by coincidence (some idiosyncratic algorithm or method that's technically wrong, but happens to work for the worksheets). The same thing happens with language. If people only encounter an idiom in a limited context, they may come to misunderstand what the idiom actually expresses. In this case that it's not strictly the cardinal directions, but the direction of the rivers.
While I agree that many people use north=up/south=down, I'm not convinced that it's meaningful to talk about the 'modern frame of reference' in this way.
We have one major river in the US which flows south, the Mississippi. It's such a big influence on the local views that people in New Orleans may use "north" to mean "up river" even in a bend where the river flows from geographic south. The Mississippi is very influential in US history. Northern slavers used the threat to "sell down the river" to the South.
We also have the Hudson River, which is important for New York. Someone sent "up the river" was being sent to the Sing Sing Correctional Facility, thirty miles north of New York City.
So it's not surprising that those set a baseline for a lot of people.
But if you're from Portland, OR, then the river flows (locally) west. If you're from Jacksonville, FL, the river flows (locally) east, after coming from the south. If you're from Nebraska, the Platte flows mostly east. In Boston, the Charles flows mostly northeasterly. (And to double-check, the Upper Charles River Reservation is indeed 1) upstream, and 2) southwest of the mouth.)
To say that the modern frame of reference has changed, I would want to see evidence for change. Where is the "upper" part of a river system used for something other than upstream?
Otherwise, my belief is that eponeponepon is asserting a personal view that is not really backed by the evidence.
My point was that upper and lower seem to (anecdata) be disconnected from the context of rivers. Upper and Lower Egypt make sense once reminded of this, but for many (anecdata) it seems upper and lower mean north and south so their intuitive sense is that the naming is "wrong" or seems off.
Yes, I think I understand your point. Yes, there are many people who use "upper" and "lower" as rough equivalents to "north" and "south".
Here is but one of many examples - "Up and down can be confusing when one talks about Egypt. In ancient times the term Upper Egypt actually meant heading downwards into Africa, while Lower Egypt meant heading upwards on the map, to the Nile Delta." http://motherdaughterbookreviews.com/guest-post-the-magic-of...
My point, however, is that the phrase 'modern frame of reference' is meaningless. At best it's "personal sense of reference" or "a common mistake."
That is, our 'modern' use now seems to be identical to what it was when the terms "upper" and "lower" Egypt were coined.
Also, note that that book review is wrong. At least, according to Wikipedia, 'Lower Egypt was known as Ta-Mehu which means "land of papyrus"' and 'Upper Egypt was known as Ta Shemau which means "the land of reeds"', so it's not like modern English uses a direct translation of the Ancient Egyptian.
So, when was this earlier time that's supposed to be different from now?
That it's actually referring to upriver/downriver in the case of Egypt is lost on those who aren't more closely familiar with the geography. It makes sense once explained, but it's non-obvious if you've become accustomed to up and down being synonymous with North and South.
Side note: There's a discussion about Common Core math and a comment by DanBC about how students may correctly fill out many math worksheets without developing any actual understanding just by coincidence (some idiosyncratic algorithm or method that's technically wrong, but happens to work for the worksheets). The same thing happens with language. If people only encounter an idiom in a limited context, they may come to misunderstand what the idiom actually expresses. In this case that it's not strictly the cardinal directions, but the direction of the rivers.