# FORCE--Landsat + Sentinel-2 Analysis Ready Data and Beyond
## 2 Product Level and Data Cube Definition  
Remote sensing products are grouped in a hierarchical classification scheme [15]. The lowest available level is commonly Level 1, i.e., radiometrically calibrated and georectified data. Level 2 data most notably include some sort of atmospheric correction. Level 3 data are temporal Level 2 aggregates that are provided in a different spatial reference, commonly a grid system with a single coordinate system. Level 4 products are model output, often derived from multi-temporal or multi-sensor measurements. In this paper, Levels 1 and 2 are referred to as lower-level products and Levels 3 and above as higher-level products, respectively. Several modifications to this scheme are commonly used. As an example, Level 3 products are the first that are mapped on a regular grid, whereas the lower-level products are still in georectified swath geometry (e.g., the Landsat Worldwide Reference System 2 (WRS-2) path/row system). In contrast, the key element of ARD is to provide gridded data [13; 16; 17]--regardless of product level. This is for e.g., reflected in ESA's production and distribution strategy of Sentinel-2 data as they already include gridding on Level 1 [6]--although still using local Universal Transverse Mercator (UTM) zones with a substantial amount of redundant data between overlapping and neighboring tiles.  
As such, FORCE adapts gridding on Level 2, i.e., all generated products are reprojected into one coordinate system (e.g., a continental projection as in [13] or [18]), and organized in smaller tiles. The following terms are defined; see Figure 1 for a graphical representation of these concepts:  
* The 'grid' as the regular spatial subdivision of the land surface in the target coordinate system.
* The 'grid origin' is the location, where the tile numbering starts with zero. Tile numbers increase toward the South and East. Although not recommended, negative tile numbers may be present if the tile origin is not North-West of the study area.
* The 'tile' is one entity of the grid, i.e., a grid cell with a unique tile identifier, e.g., X0003_Y0002. The tile is stationary, i.e., it always covers the same extent on the land surface.
* The 'tile size' is defined in target coordinate system units (most commonly in meters). Tiles are square.
* Each 'original image' is partitioned into several 'chips', i.e., any original image is intersected with the grid and then tiled into chips.
* Chips are grouped in 'datasets', which group data according to acquisition date and sensor. Each dataset contains several 'products'. At minimum, a reflectance product and an accompanying quality product are generated.
* The 'data cube' groups all datasets within a tile in a time-ordered manner. The data cube may contain data from several sensors and different resolutions. Thus, the pixel size is allowed to vary, but the tile extent stays fixed. The data cube concept allows for non-redundant data storage and efficient data access, as well as simplified extraction of data and information.  
Figure 1: Overview of gridding and data cube terminology used in FORCE (Framework for Operational Radiometric Correction for Environmental monitoring).