Rasters

In GeoTrellis, the GIS notion of a Raster is represented as a case class (basically, just a labelled tuple) with something implementing CellGrid (almost always Tile) on one side and an Extent on the other. The CellGrid represents data on the surface of a sphere while the Extent specifies how those values map onto said sphere (in an appropriate projection; see map projections for more).

import geotrellis.raster._
import geotrellis.vector.Extent
// The tile
val tile = ArrayTile(1 to 100 toArray, 10, 10)
// and its spatial extent
val extent = Extent(0, 0, 1, 1)
// jointly form a raster
val raster = Raster(tile, Extent(0, 0, 1, 1))

TODO: image/sketch/diagram of Raster components

A Raster, having both a grid of cells and a specification of the region that grid represents, is able to calculate the distance each cell in a CellGrid must cover to satisfy the width and height of its corresponding Extent. In practice this means that, when resampling or reprojecting, a Tile (which holds its data without regard to that data's extension in the real world) doesn't have enough information to get the job done; these cases generally demand a Raster or else require an Extent from which a Raster can be built.