Baldur's Gate 3

It could be 120ft±2.5ft without changing anything about game mechanics, but introduces at least 2% error already.

To measure 100 miles, assuming your 120 foot measurement and assuming that game mechanics = in-world logic, you'd need to cast your spell 4400 times per 100 miles, moving at a glacial pace as you measured. You'd also need to ensure that your path was perfectly straight, you accounted for elevation changes, that you precisely marked each endpoint, etc etc etc.

If you're seeking accuracy, measuring the outside of a 200mi x200mi square would require doing all those calculations across 35,200 castings. If you were measuring the internal dimensions to about a one mile accuracy, you'd be in the millions of castings and you'd still need to ensure you accounted for all of the above.

Regardless, we know that it doesn't work that way because FR sourcebooks tell us that maps are inaccurate, rare, and expensive. We know that the maps are inaccurate because they've told us so!

@optissima @GBU_28 don't want to necropost, but just stumbled upon that discussion.

I take the distance in the rule more like game mechanics.

If we take the description of the magic in the FR, I would say distance depends on the density of the threads of the weave, the dead magic area, etc. So not very dependable.

On the wish spell, I could agree though, that you could get more accurate info via this.

Some good points. I'm just going to continue this discussion because it's interesting and it helps me prepare my games to consider these things.

1: Universal answers don't necessitate universal acceptance, and it can make for more interesting lore when that's the case. As an example: in the lore of Legend of the Five Rings, it's common knowledge in the Empire that the official map of the Empire has a massively inconsistent scale, with journeys of similar charted length having up to a threefold difference in travel time. Savvy travellers know to plan accordingly, but no one would ever question Imperial doctrine, as the charting of the Empire was an act of a very real and tangible living god. This is where I got my praying at every temple comment; it's common for people to avoid accidentally badmouthing the Empire by saying "I took longer than expected as I took every opportunity to honour my ancestors at every shrine on the road."

1b: The pantheon of the Forgotten Realms is ever expanding and there are gods in that pantheon that are opposed to Mystra, as well as luddite gods who are oppose the gods of innovation such as Gond. Gondians certainly promote the advancement of science, and Mystrans and Oghmans promote the advancement of magic to a very certain extent, but there are gods in the pantheon who would task its worshippers with direct opposition of these missions, if for nothing else than to piss off their rival god.

2: This comes back to point 1. Different states will have their own standards of measurement, often using the same name, and the usage of these standards are very often more political than logical. A famous example from history is Napoleon's height. Napoleon was "5 foot 2 by the French measurement and 5 foot 6 by the English measurement," which made him a French adult man of average height. It was a common political tool to report him in the British Press as 5 foot 2, thus implying that he was short of stature.

Imagine the compounding issue of different species interacting in the Sword Coast. A human-majority patriarchal city state may define an inch as the average length of the the second knuckle of an adult human male's middle finger, while an elvish-majority patriarchal enclave may define it exactly the same but for an elf's finger. These slight discrepancies aren't an issue until they can be exploited for political gain; an elvish embassy may be established at a distance no closer than a mile to the Palace of the Magistrates, but there's roughly a 10 human-yard difference between an elvish mile and a human mile.

If someone casts a spell asking for a measurement and they are told "10 miles," is that 10 miles from their perspective, 10 miles from the perspective of whoever invented the measurement spell, 10 miles according to some third "universal" perspective, or something else entirely?

3: Again from my previous comment, the precise limitations of spells are assumptions and generalisations made for the purpose of codifying into a game. In the actual fiction, spells are quite variable dependent on the caster and their abilities. The only general assumption we can actually make is that a set of repeatable actions yield roughly the same result: if you rub a glass rod with a bolt of fur and sing the chorus of Tubthumping backwards, lightning appears. The reason that in the current edition of the game we have somewhat concrete descriptions of spells is that we as the players require a certain level of abstraction in order to play the game; The GM shouldn't need to have an idea of wind speed, the aerodynamics of the flier, and all other forces in order to make a quick decision to determine how the flier flies. Some randomness of outcome is still evident on the modern game rules, such as the damage from spells being random and spells like sleep affecting a random number of creatures. Older editions were a lot more meticulous with this.

Edit: specifically tackling Wish, assuming even a perfect casting would not yield a perfect map. Check out the Coastline Paradox for a real world example of how natural bodies such as coastlines fail to have well-defined length. No amount of arbitrary precision measurement is going to change the facts that coastlines and waterways have fractal dimension.

4: At least in 5e rules as written (and I dislike this and usually houserule it when forced to play D&D), with the exception of protection scrolls, reading a spell scroll requires caster to have the given spell on their spell list.