World files for raster datasets
Last modified November 12, 2009
Print all topics in : "Properties of raster data"
This topic was updated for 9.3.1.
Images are stored as raster data in which each cell in the image has a row and column number. Vector data, such as feature classes in geodatabases, shapefiles, and coverages, is stored in real-world coordinates. To display images with this vector data, it is necessary to establish an image-to-world transformation that converts the image coordinates to real-world coordinates. This transformation information is typically stored with the image.
Some image formats, such as ERDAS IMAGINE, BSQ, BIL, BIP, GeoTIFF, and grids, store the georeferencing information in the header of the image file. ArcGIS uses this information if it is present. However, other image formats store this information in a separate ASCII world file. World files can be created with any editor, or you can export a world file for raster datasets using the Export Raster World File tool.
It's easy to identify the world file that should accompany an image file; world files use the same name as the image, with the letter w appended. For example, the world file for the image file mytown.tif would be mytown.tifw, and the world file for redlands.jpg would be redlands.jpgw. Often the naming convention varies from this first description, using the first and third characters of the image file's suffix and a final letter w as the world file suffix. Therefore, for mytown.tif, the world file would be mytown.tfw, or for redlands.jpg, its world file would be redlands.jgw.
For images that lack an extension or have an extension that is shorter than three characters, the letter w is added to the end of the file name without altering it. Therefore, the world file for the image file terrain would be terrainw and the world file for the image file floorpln.rs would be floorpln.rsw.
|Raster data file||World files|
|image.tif||image.tfw or image.tifw|
|image.bil||image.blw or image.bilw|
|image.jpg||image.jgw or image.jpgw|
The image-to-world transformation is accessed each time an image is displayed, for example, when you pan or zoom. The transformation is calculated from one of the following sources, listed in order of priority:
The contents of the world file will look similar to this:
20.17541308822119 0.00000000000000 0.00000000000000 -20.17541308822119 424178.11472601280548 4313415.90726399607956
x1 = Ax + By + C y1 = Dx + Ey + F
x1 = calculated x-coordinate of the pixel on the map y1 = calculated y-coordinate of the pixel on the map x = column number of a pixel in the image y = row number of a pixel in the image A = x-scale; dimension of a pixel in map units in x direction B, D = rotation terms C, F = translation terms; x,y map coordinates of the center of the upper left pixel E = negative of y-scale; dimension of a pixel in map units in y direction
NOTE: The y-scale (E) is negative because the origins of an image and a geographic coordinate system are different. The origin of an image is located in the upper left corner, whereas the origin of the map coordinate system is located in the lower left corner. Row values in the image increase from the origin downward, while y-coordinate values in the map increase from the origin upward.
20.17541308822119 - A 0.00000000000000 - D 0.00000000000000 - B -20.17541308822119 - E 424178.11472601280548 - C 4313415.90726399607956 - F
ArcGIS will automatically read the georeferencing in the header information of some file formats, such as GeoTIFF, which will override any georeferencing information stored in a world file. You can change this by checking the Use world file to define the coordinates of the raster check box on the Options dialog box.