Rasters contain a set of metadata that defines the properties for the raster as a whole. These properties include:
- number of bands (channels or layers)
- number of rows and columns (lines and pixels)
- cell size (spacing)
- cell origin
- Ground Control Point (GCPs)
A cell is the rectangular area created in the x and y dimensions by the spacing of pixels from the raster origin.
Spacing or cell size is the fixed distance in the x and y dimensions between each pixel in the raster. Some formats store only one spacing value, meaning that it must be the same for both the x and y dimensions – this is often referred to as square cells.
The raster origin is the upper left x and y of the raster at which the coverage of the data sample begins. Specifically, the raster origin in FME is the upper left corner of the upper left cell in the raster.
Cell origin is the point within each cell of a raster from which the pixel for that cell is derived. The lower left corner of the cell in the x or y dimension is 0.0, while the upper right corner is 1.0. A cell origin of 0.5 in x and 0.5 in y would put the data point for each cell in the center of the cell, which is the default representation in FME.
Extents or bounds for a raster are represented by the lower left ground coordinate and the upper right coordinate covered by the raster data. This is sometimes referred to as cell bounded. The minimum x and maximum y values that comprise the upper left corner of the raster extents are equivalent to the raster origin.
Rotation is used to represent rasters that are not aligned with the x and y axes. The x rotation is the angle in radians as measured in a counter clockwise direction from the positive x axis. The y rotation is the angle in radians as measured in a counter clockwise direction from the negative y axis. Note that a different rotation from the x and y axes produces a shear. The rotation point is the top left corner of the top left cell of the raster.
Ground Control Points, or GCPs, may also be present in the geometry of a raster. If present, these refer to a set of points used to georeference image or elevation data, with each point 'tying' a row and column location in the raster to an x,y location on the earth. A coordinate system will also be present in the properties of a raster containing GCPs, as opposed to being stored on the feature itself. GCPs can either be applied to the raster resulting in the image being georeferenced and tagged with the GCP coordinate system, or the GCPs can be extracted and stored on the resulting data file for those formats supporting unreferenced data and GCP storage.