Images are stored as raster data where each cell in the image has a row and column number. In addition to these ordered rows and columns of pixels, real-world coordinates are also needed. To display images in the correct location, 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.
Generally image formats, such as grid, TIFF, BSQ, BIL, or BIP, store the georeferencing information in the header of the image file. However, several image formats store this information in a separate ASCII world file. Where the georeferencing information is stored often depends on the capabilities of the software used to generate the files or the user's preference. World files can be created with any editor, or you can export a world file for a raster dataset using the Export Raster World File tool.
World file naming conventions
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. Sometimes 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.
Examples of world file names
|Raster data file
image.tfw or image.tifw
image.blw or image.bilw
image.jgw or image.jpgw
World files and georeferencing
If the transformation cannot be expressed as a world file, the transformation information will be written in the .aux.xml file and also in the world file, as an approximate affine transformation. This world file that is created will have an x on the end of the extension name. For example, a TIFF image with an approximate affine transformation has the extension .tfwx. Keep in mind that this is not an exact transformation; it is only an approximation.
There is a special case where this text file contains a true affine transformation. If using the Save option on a raster dataset that already contains map coordinates, a text file with the x extension is written. For example, if georeferencing is performed on a TIFF image that already contains map coordinates, then a .tfwx file that contains a true affine transformation will be created.
How georeferencing information is accessed
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 several sources:
- The header file (if the image type supports one)
- The world file (used first if the Use world file to define the coordinates of the raster check box is checked on the Options dialog box)
- The row and column information of the image (an identity transformation)
World file contents
The contents of the world file will look similar to this:
When this file is present, ArcGIS performs the image-to-world transformation. The image-to-world transformation is a six-parameter affine transformation in the form of
x1 = Ax + By + C
y1 = Dx + Ey + F
x1 is the calculated x-coordinate of the pixel on the map
y1 is the calculated y-coordinate of the pixel on the map
x is the 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 and D are the rotation terms
C and F are the translation terms; where the x,y map coordinates of the center of the upper left pixel
E is the negative of y-scale; dimension of a pixel in map units in y direction
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.
The transformation parameters are stored in the world file in this order:
20.17541308822119 - A
0.00000000000000 - D
0.00000000000000 - B
-20.17541308822119 - E
424178.11472601280548 - C
4313415.90726399607956 - F
Each of these coefficients can be defined as
A = mx · cos t
B = my · (k · cos t - sin t)
D = mx · sin t
E = -1 · my · (k · sin t + cos t)
C = translation in x direction
F = translation in y direction
mx = change of scale in x direction
my = change of scale in y direction
k = shear factor along the x-axis = tan (skew angle, measured from the y-axis)
t = rotation angle, measured counter-clockwise from the x-axis
Set ArcGIS to read the world file first
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 window.
- Click the Project tab.
- Click Options.
- In the Options window, click Raster and Imagery.
- Expand the Raster Dataset category.
- Check the Use world file to define the coordinates of the raster check box.
- Click OK..