I've been creating quite lengthy new map series for both the Nations and the History of Forgotten Realms, so this is a topic I've delved into quite a lot over many years.
The OG map is Ed Greenwood's, that he had created in the 1970s and redrew circa 1986 to send to Jeff Grubb at TSR. This is the infamous "map that covered the entire office floor and blocked access to the women's toilets," consisting of 50 or so smaller maps that had to be combined into the megamap. This map was subsequently drawn over with Doug Niles' version of the Moonshaes replacing Ed's originals, and then the SW extent of the Great Glacier being rolled back and replaced by the Bloodstone Lands from the hitherto unrelated adventure path. TSR then took that revised map to create the 1E Faerûn large map (1"=90 miles) from the Old Grey Box. This map, and the subsequent 2E version, both had to omit the south coast of Faerûn due to the scaling issue the 3E designers later complained about, namely that Faerûn is awkwardly proportioned to fit onto standard poster maps. Both maps terminated just south of the tip of Chult and just east of Thay, so almost the entire Shining South + Shining Lands + Chultan Peninsula + Raurin + Semphar/Murghôm regions are left out. Intriguingly, the very first Realms novel,
Darkwalker on Moonshae, does have a complete map of all of Faerûn incorporating these areas, but it's so zoomed out that almost all fine detail is missing.
The large map was then drawn over again at 1"=30 miles to produce the Western and Eastern Heartlands maps from the Old Grey Box, plus the larger maps from the FR line. I recently took scans of all of the 1"=30 miles maps from the FR line and merged them together to find that they mostly matched up (gloriously so in the case of the OGB maps +
The Savage Frontier +
Empires of the Shining Sands +
Anauroch +
The Bloodstone Lands). However, they did started encountering errors when I got to the
Dreams of the Red Wizards map and then the
Old Empires map (which is at a different scale) went off the charts. As far as I can tell, there was some miscalculation or error with eastern Faerûn when drawing this zoomed in map and this resulted in mistakes which were never rectified.
You then had the addition of Kara-Tur, but Kara-Tur as originally envisaged in
Oriental Adventures (1985) was either a stand-alone thing or a remote continent on Oerth, so its size was kinda irrelevant and Zeb Cook and his team went wild with how massive it was (although it wasn't mapped in
Oriental Adventures itself, but soonafter in the first OA adventures). Even in
Kara-Tur: The Eastern Realms (1988) they didn't fully appreciate how much bigger Kara-Tur was then Faerûn because the two sides did not fully match up. It's only when
The Horde (1990) was produced specifically to link the two continents they realised that Kara-Tur was
comically too big so they reduced the size by adjusting the scale. That did result in a weird big peninsula appearing to the SW, as shown in
The Forgotten Realms Atlas (1991) by Karen Wynn Fonstad (RIP), but that proved a boon because it could be turned into Zakhara for
Al-Qadim: Arabian Adventures (1992).
By this point we were into 2E. 2E kept the geography exactly the same, but more detailed, to the point where 1E and 2E maps align perfectly with one another.
At the end of 2E we also got
The Forgotten Realms Interactive Atlas (1999) which was supposed to be the absolute last word on Forgotten Realms geography, with the first canonical world map and over 800 maps all interlocking with one another, with a lot of inconsistencies from the source maps smoothed over (unfortunately, some new ones were introduced, and the less said about the Wizards of the Coast-mandated font choices the better).
We then got the shift to 3E with this decision to shrink Faerûn to remove "dead space" (never a complaint I'd heard previously) and fit the entire map on one poster, which was maybe a more reasonable point. However, their resizing of Faerûn resulted in about a 20% net loss of land to the continent, which is a lot. As well as tilting the continent, eliminating most of the Shaar and pushing the "dead" Great Glacier off the map, they wound up apparently decanonising R.A. Salvatore's
The Cleric Quintet (Impresk Lake is now on a plain and the Snowflake Mountains are hundreds of miles away to the south), retconning Steven Schend's Erlkazar from
Lands of Intrigue out of existence (it literally just vanishes) and compressing areas of the map they weren't supposed to compress. It was a bit of a mess. It also made the literally-just-released
Forgotten Realms Interactive Atlas no longer canon, which I'm sure was enjoyed by the hundreds of people (many of them volunteer fans working for free) who'd worked on it for three years.
Amusingly, the FRIA contract extended for several years into 3E's existence, so later updates to the FRIA feature new locations added in 3E maps but "backported" onto the 1E/2E configuration of the Realms (I've done a lot of the same kind of thing on my maps, and then Mike Schley did the same thing with his 5E maps).
4E's maps are pretty poor, to be honest, but the smashing of the continent didn't really help, and there's a lot of weirdness going on with the placement of locations even beyond the post-apocalyptic vibe (i.e. some locations in earlier editions have been arbitrarily moved).
For 5E, they simply snapped back to the 1/2E configuration. Mike Schley's splendid map of the Sword Coast aligns 100% to the FRIA, and the large-scale map of the whole continent in
Sword Coast Adventurer's Guide shows everything back to where it should be. All subsequent official 5E maps have adhered mostly to 1/2E canon, apart from some scale errors in a few areas (the maps of Phandalin I think suffer a bit from this), but nothing major.
So everyone's happy and everything's fine now, right? Nope. The recently-published
Lore & Legends art book features a new map of Faerûn which at first glace is drawn from the 3E map, which confused the hell out of everyone because the reversion to the 2E config had not only been clearly shown on all previous 5E maps but also acknowledged (tongue-in-cheek) in the
Sword Coast Adventurers' Guide text. Further comparisons revealed that WotC had taken
this popular fan map (derived from 3E) and simply hired a professional artist to draw over the top of it, incorporating a lot of non-canonical elements that Ed Greenwood had shot down on Twitter (like the shape of the Alamber Sea and the horrible north coast of Faerûn). They'd clearly not taken into account any of the lore or map changes along the way. What the heck they're going to use for 5.5E/whatever moving forwards, nobody knows.
To summarise, there's an easy way to know what map you're looking at. Look for Port Belurian on the northern tip of Chult. If this port is far to the
south-west of Calimport, you're dealing with a 1/2/5E map. If it's
due west of Calimport, it's 3E. If Chult is an island, it's 4E.