The name of the spatial index is derived from the acronym (I)n(D)e(X) (P)olygons at the zone tile level, or idxpzone.
Each zone overlaps adjacent zones by 15000 feet, the dimension of two 7500 ft tile cells.
Tile identifier is the map quadrant each tile occupies, NE,NW,SW and SE, with expanded zone_names: northeast, northwest, southwest and southeast, respectively. When used as part of a file name the tile identifier is prefaced with a 'z' indicating that its extent is derived from the idxpzone scheme, i.e, zne, znw, zsw, zse.
Data at this tile level is often stored at a lower resolution and/or higher compression ratio than the same data set stored at a lower tiling level, but provides the opportunity for users to choose the tile form of the data most appropriate for their needs.
As each cell is defined with orthogonal edges relative to its underlying idxp7500 tiles, it accomodates most common pixel resolutions of multiples sample distance (gsd) without partial pixel effects and without zero or NULL data 'splinters'.
Not all image and raster data is stored at the tiling level represented by this index. Data is stored at this level if full-resolution, uncompressed data would generate larger than manageable file sizes at a smaller tile size. Alternatively, most data available at this level is also found at the primary user access tile level - idxptrmbr - based on the Public Land Survey System (PLSS) township-range grid.