What is Raster data?
A. Raster data is comprised of a series of cells, which together form an overall picture. It is a means of storing and displaying maps in digital format and is often produced either directly from the vector map or by scanning the original paper documents. The raster data can be positioned on the Irish Grid within a Geographic Information System by using supplied TIFF World Files (or twf files). TWF files contain information about the origin point of the raster file and the individual cell size. Each raster file is divided into a series of rows and columns forming a grid. Each cell, within that grid, is rectangular in shape. The example below shows a sample .twf file for OSNI 1:10,000 raster data. 0.63324538258575 Cell size in the X plane (metres) 0.00000000000000 Rotation in the X plane 0.00000000000000 Rotation in the Y plane – 0.63347520538454 Cell size in the Y plane (metres) 284000 Irish Grid Easting co-ordinate 386400 Irish Grid Northing co-ordinate The sign of the ‘cell size in t
