My observations and suggestions to terrain builders:
For all intents and purposes, the resolution of everything in the new terrain engine is at 660', or 201 meters. This affects elevation, water and ground texture mapping.
A square of 660' in the terrain is made of two right triangles with their mutual hypotenuse running from NW to SE. This impacts ridgelines to a minor degree, but the biggest impact is on the placement of water and shorelines. So long as these factors are kept in mind, a 1320' wide channel can be drawn either NS or EW. A very narrow channel may be drawn from NW to SE but drawing a narrow channel from the NE to the SW is problematic and probably to be avoided. Fortunately, the new rivers under development should solve most if not all of these problems.
The resolution for ground textures match the 660' resolution, meaning a 660' square is the smallest bit of a texture you can draw, and they will bleed-blend into some of the surrounding 330' spaces.
As previously mentioned, the tiles in each Atlas are currently drawn in sequence, meaning there can be a band of every terrain type between adjoining textures. This can be mitigated within texture sets. See jpg's.
If placing textures Atlas 0 Type 0 next to Atlas 0 Type 15, there is normally 15 intervening bands of texture crowded into a 330' border, but if the texture in Atlas 1 Type 16 (currently farm) is neutral to all or most of the textures in Atlas 0, then any intervening bands can disappear. By neutral, I mean reasonably bare grassland for example.
When a 660' band of Atlas 1 Type 16 is placed between Atlas 0 Type 0 and Atlas 0 Type 15, the only band will be for the Atlas 1 Type 16 and if it's neutral, it will blend in with both Atlas 0 textures without the 15 intervening borders.
This works in reverse as well. If Atlas 0 Type 0 (currently beach sand) is neutral, it can be used to blend between two Atlas 1 types. Again, see the jpg examples.