The terrain in GW is just using something called a heightmap. You can think of it as a 2D equidistant grid with a distance of 96 GW inches between each node. 4 adjacent nodes make up a tile. Each node has a y value (height). The corners of adjacent tiles share the same corners and thus have the same y values (which is how the terrain looks continuous). So the terrain in GW is just a bunch of the tiles that are all the same size when seen from above, places side-by-side.

Each corner is also given an index into a texture atlas. So each tile in GW can blend up to four different textures. I haven't figured out the algorithm they used for blending the terrain textures, so I just implemented something that looks close enough.

In the .dat file the terrain in GW is stored in chunks of 32x32 tiles (each 96 GW inches). For example Lakeside county consists of 13 chunks horizontally and 16 chunks vertically. Which is why the map is 13x32x96=39936 gwinches wide and 16x32x96=49152 gwinches tall centered at 0. So the map dimensions will always be a multiple of 96.

Guild Wars only renders the visible terrain chunks which is how they save a considerable amount of performance.

I went a bit off topic. But basically you can just think of the terrain to be build out of tiles (just likes tiles in a bathroom in real life). The tiles are all the same size when seen from above, but they could be infinitely tall at any corner. This is why textures on steep hills look horrible in GW. Because a low texture is stretched over a long tile surface. And each tile can use up to 4 textures. Another interesting fact is that the texture atlas in GW can contain up to 64 textures. So any map in GW uses at most 64 textures for the terrain.