Image and raster data management > Properties of raster data
World files for raster datasets |
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Release 9.3
Last modified November 12, 2009 |
Print all topics in : "Properties of raster data" |
Note:
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.
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. 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 |
| image.raster | image.rasterw |
| image.bt | image.btw |
At ArcGIS 9.2 SP2, if the transformation cannot be expressed as a world file, Update Georeference writes the transformation into the .aux.xml file and also writes an approximate affine transformation into a text file or a world file with an x on the end of the extension name. For example, a TIFF image with this 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 performing Update Georeference 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 that already contains map coordinates, the Update Georeference command creates a .tfwx file that contains a true affine transformation.
This text file can be used as a world file if it is needed for ArcGIS 9.1 and third-party software. To use this text file, delete the x in the world file extension name. Since this text file is not part of the raster dataset in ArcGIS 9.2, it should not be used as a world file in versions where ArcGIS can read the .aux.xml file such as in ArcGIS 9.2 and beyond.
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 the following sources, listed in order of priority:
- The header file (if the image type supports one)
- The world file (This will be used first if the Use world file to define the coordinates of the raster check box is checked on the Options dialog box.)
- From the row/column information of the image (an identity transformation)
World file contents
The contents of the world file will look similar to this:
20.17541308822119 0.00000000000000 0.00000000000000 -20.17541308822119 424178.11472601280548 4313415.90726399607956
When this file is present, ArcView 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
where
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.
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
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 dialog box.
- Click Tools on the Main menu and click Options.
- Click the Raster tab in the Options window.
- Within the Raster tab, click General, and check the Use world file to define the coordinates of the raster check box.
- Click OK.