Currently extended to include all townships within or partially within King County as well as those townships in the southwestern portion of Snohomish County included within King County's ESA/SAO project area.
The name of the spatial index is derived from the acronym (I)n(D)e(X) (P)olygons for (T)ownship-(R)ange, (M)inimum (B)ounding (R)ectangle, or idxptrmbr.
Tile label is the t(township number)r(range number)as in t24r02. The meridian zone identifiers, N for townships and E for range is inferred as this index is intended as a local index for ease of use by the majority of users of GIS data. Lowercase identifiers are used for consistency between Unix and Windows OS storage.
This index or tile level is the primary user-access level for most LiDAR elevation, orthoimagery and high-resolution raster landcover data. However, not all image and raster data is stored at the tiling level if a given data's resolution does not justify storing the data as multiple tiles.
The index also serves as a useful central reference scheme for identification of real-world locations as it references the commonly used township-range grid of the PLSS.
As each cell is defined with orthogonal edges relative the Washington State Plane coordinate system, and is defined by easting and northing bounds rounded to the nearest 100 foot, it accomodates most common pixel resolutions of multiple Ground Sample Distance without partial pixel effects and without zero or NULL data 'splinters'.