Tile LoD Area Coverage Rules Tile-LOD Hierarchy Rules Tile-LOD Replacement Rules

39 © 2016 Open Geospatial Consortium

2.1.2.1 Tile LoD Area Coverage Rules

Requirement 14 http:opengis.netspecCDB1.0core lod-area-coverage A tile from LOD –10 to LOD 0 SHALL occupy the area of exactly one CDB Geocell. This is true for all CDB Geocells from all CDB Zones. Starting with LOD 1 and up, this area SHALL recursively be subdivided into smaller tiles, every level corresponding to a finer representation of the previous level allowing for multiple levels of detail. Consequently, tiles at a given LOD never overlap with others of the same LOD, and are always aligned with at least two of the edges of tiles at the previous LOD. Figure 2-4 illustrates how different Levels of Detail LOD’s can be combined within individual geocells. In the illustration, some CDB Geocells of the pictured earth slice are represented using five LODs, while others have only three or four LODs. Each Geocell included in a CDB data store shall represented by an instance of at least the coarsest tile supported, i.e., one tile at LOD −10. In addition, it is not necessary that the entire geocell be represented at the same level of resolution across the entire cell. However, if a tile is present at LOD n, it implies that a courser tile exists at LOD n−1 covering the area of the tile at LOD n, until LOD −10 is reached 40 © 2016 Open Geospatial Consortium Figure 2- 4: Earth Slice Example Five Levels of Detail

2.1.2.2 Tile-LOD Hierarchy Rules

Requirement 15 http:opengis.netspecCDB1.0core hierarchy The LOD hierarchy of all tiled datasets SHALL be complete for every CDB Geocell. However, each CDB Geocell may have a different number of Tile-LODs. 41 © 2016 Open Geospatial Consortium

2.1.2.3 Tile-LOD Replacement Rules

In general, finer tiles replace coarser tiles. The requirements are: Requirement 16 http:opengis.netspecCDB1.0core tile-lod-replacement For negative levels of details, a tile at LOD n SHALL replace exactly one tile at LOD n−1 since both tiles cover the same area. For positive levels of details, a tile at LOD n SHALL be replaced by 4 tiles at LOD n+1 since tiles from LOD n+1 cover only a quarter of the area covered by the tile at LOD n. In the case of positive LODs, note that it is not necessary that all 4 tiles from LOD n+1 exist; as long as one of the four tiles is present, the replacement of the tile at LOD n can take place. For instance, one tile at LOD −1 is replaced by one tile at LOD 0 which is in turn replaced by four tiles at LOD 1. The replacement of coarser tiles with finer tiles stops when no finer tiles exist.

2.1.3 Handling of the North and South Pole