I have watched each of the films and made tiny notes at crucial points in an attempt to draw a map of the Great Valley and the surrounding areas. I haven't yet uploaded a map, but I will draw a text description.
Lands Around GV:
N: Mountains
NE: Water from 3 (within Mts), Barren lands/Berry Valley.
E: Barren Lands from 1.
SE: Smoking Mountains.
S: Smoking Mounts and desert.
SW: Desert, then Land Of Mists)
W: Desert, and Movie 5 lands
NW: Mountain routes, and Movie 9 lands.
(That was written presuming the mountain chain runs N-S. Looking at maps, and judging by scenes in 1 and 4, the chain may really run NNW-SSE.)
Now follows my reasoning. (WARNING: LENGTHY NOTES FOLLOW) Please note, since some movies show areas that don’t necessarily fit together, I have changed a few things to fit them better. For example, Sun placement has not always been noted.
First of all, what we know from the first film is that the eastern lands are barren, the north and south are divided by a large chasm, and a large mountain chain runs north-south in the west. This is where the great valley is.
Now, lets look at the main dinosaurs, and where they were found. Personally, I think the GV was located in Western America, as an older feature of the Rocky Mountains. Look at this map:
US Cretaceous MapTaking some liberties with the surrounding areas, that could easily be the world of LBT.
Triceratops was only located in Western America. Apatosaurs, Stegosaurs and Pteranodons were also found here. Saurolophus was too, but the Asian species which had a longer crest fits Ducky's look better. However, there was a land bridge connecting Asia and America in the north, so her family could have migrated.
Now, lets look at the knowledge that we got in each film.
In the first film, we know that there are two mountain lines to the east. One is the barrier that stops Sharptooth following them. Behind this is the Longneck statue. Then, there is the other mountain line that Littlefoot pushes Cera down to fight in. Over the hill down there is the Great Valley. To the left (South) are "Mountains That Burn"; these are presumably the same as the Smoking Mountains. The tiny lake that they defeat sharptooth at is located near the GV (You can see the rock on the plateau over the lake when Littlefoot sees his mother in the clouds. The film originally had this scene earlier.) As for the Great Valley, one thing that must remain consistent, is that a waterfall flows into the valley, and a river runs from it, East To West.
The second film doesn't show much outside the valley, apart from the large area of tar and skeletons which appears to be the same place as the smoking mountains in the first film. As for the route that is taken to get there, it looks as if they followed Ozzy and Strut over the mountains surrounding the GV, travelling south towards the Smoking Mountains, before returning through the area created by the rockfall; a passage to the east.
The third film gives the waterfall a name; The Thundering Falls. When the trapped water is found, it takes up a large area (this may be flooded land). It must be near the falls, and the land to the southeast has been mapped, that means that the water must be to the northeast, probably further north than east, to provide room for the adults to reach the valley (In the first movie)
Four and Five are a bit confusing. Four shows the valley emerging from the mountain chain into a desert peppered with rocks. Across from these (Apparently West) are high rocks that border the Land Of Mists.
Five also shows a vast area of desert, although in some areas it appears to be desolate land. This could be east, but I doubt that the characters would ever return there, and also in Nine, Cera remarks that they went “Right” at the Smoking Mts. The explanation? The land is desolate, recently wrecked by the locusts.
Six shows the area where Saurus Rock is located. This is an interesting area. First of all, in Grandpa Longneck’s story, he claims that Saurus Rock, and the area where the attack took place, were a part of the Great Valley. If you ask me, it once was an area of the valley, but was abandoned for lack of safety, or a later event that made it unsafe, such as an earthquake. For a mountain line that big, plus with volcanos, the GV must be on a faultline (This could be the San Andreas Fault, following my Western America theory).
Seven is interesting as it shows that the GV is not completely bordered by mountains. The GOF cross a gorge over a river that divides the Valley and the Smoking Mountains, and Threehorn Peak. The river is seen to go over into fertile lands. Presumably, this area, and Threehorn Peak, are to the south, past the dangerous areas of the first film.
The flashback in their film is also interesting. Presumably, the great divide stretched all the way to the mountains, otherwise the families could have reunited easily. Pterano leads his group to a small canyon in the south (left) while the others continue their trek (right).
Eight shows that there is a mountain route to the valley. Since all other directions have already been shown, this must be a northern route. There is also a tall mountain that is apparently used as a landmark. This would make up part of the western wall of the GV.
Nine shows more waterfalls, but this is a result of the rains. Interestingly, an Earthquake destroys one small part of the Valley, creating a gorge. As for the route, Presumably, the GOF are travelling west with Mo.
Ten is interesting, as their route appears to go through the Land Of Mists. Both 4 and 10 go through a high rocky area, and then a misty swamp. The longneck herd in movie 10 go further. This places the LOM to the SW. Further south is the longneck canyon.
11 shows nothing new, just the same dead lands of the smoking mountains. Presumably, their cave is located to the SE.
12 is just a barren land, that could be many areas. Since they go past an active volcanic area, it is probably south, or possibly the area created by the earthquake in 9.
13 is interesting. The land we see when they first enter the mysterious beyond doesn’t match any of the sequels yet, so it must be the lands to the north of the divide. Berry Valley is located far to the north, though slightly to the east of the GV.
Okay, that’s enough for now. I have more, but it can be difficult adapting my notes to computer form. I’ll post more later.