ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS", Bangkok, May 23-25, 2001 
4. The Technology of Tile-to-Tile Edge Match [Willis 98] 
For multiresolution spatial data management and calculation, 
spatial data are huge. If we calculate all available data same 
time, no doubt it is time resuming process. So we must look for 
practice methods. This affect is normally achieved by divideing 
the terrin up into retangular grid of tiles. Each tile can have 
multiple LOD representations, and can switch between these 
representations independently. Tiling schemes come in two 
standard flavors: "replacement LODs”, where the all LOD 
representations for a tile are the same size; and “Quadtree 
LODs", where the current LOD tile is quartered at the next 
LOD into four smaller tiles. In order for the grid of tiles to 
appear as a continuous surface, each tile must match its 
neighbor along their shared edge, even when the neighbor is 
displaying a different LOD representation. This is typically 
accomplished by forcing the geometry of the shared edge to 
remain constant throughout all LOD representations of a tile. 
If the border never changes, it will always match the neighbors. 
But this solution comes at the expense of the visual quality of 
the terrain skin. Because the edges of the tiles must remain 
the same, the number of points in the edge cannot be 
appropriate for the coarsest LOD and the finest LOD at the 
same time. You will either have too many points for the 
coarsest LOD, or too few points for the finest or both. This 
uneven distribution of points and polygons along the edges of 
the tiles results in odd triangulations that are visible in the final 
database, thus drawing attention to the boundaries between 
the tiles. Because this side-effect becomes more pronounced 
as the number of LODs increases, terrain database developers 
typically limit themselves to only a few LODs, even when using 
more would provide better performance. And because 
quadtree LODs makes this side affect even more noticeable, 
terrain database developers typically avoid quadtree LODs 
Fig. 11 Multiple levels-of-detail (LOD) tile 
Fig.9 Terrain databases are typically 
layed out on a rectangular grid. 
Each rectangle of terrain is called a tile 
Least 
Detail 
Fig. 10 The LODs create distracting 
Medium 
Detail 
Most 
Detail 
To solve above, A tile can be partitioned into 5 parts: an interior, 
and 4 seams, each of which touch one edge of the tile 
perimeter. Any seam can be swapped out for a different 
representation, as long as the inner edge of the seam remains 
the same. Multiple representations of a seam allows a single 
tile to match to different LODs of its neighbors, while the 
number of points along the edge of the tile changes with each 
LOD. Each seam has multiple LOD representations, one to 
match each LOD representation of the adjacent tile. The seam 
acts as a transition zone between LODs. If the seams are 
LOD-switched properly, the edges between tiles can change 
while still maintaining an unbroken surface. 
Both Tiles at Saine Detail 
Fig. 12 Tile is divided five areas. 
Fig. 13 Seam Substitution 
Fig. 14 Tile Mixing and Match 
371