Raster data, or images, such as GIFs, JPEGs, MrSIDs, or TIFFs, are positioned in a view based on the attributes of the image pixels. To overlay raster data with real world vector data, images must be georeferenced to the same coordinate system.
Image formats may store georeferencing coordinates in the header of the image file (Erdas IMAGINE files, MrSID images, GeoTIFFs, and so forth)or in an additional file called a header file .hdr (BIL, BSQ, and BIP). For example the header of the image myimage.bil would be myimage.hdr.
If the image does not contain header information, it can be georeferenced using a world file. An image world file provides coordinates and dimensions for the upper left pixel of the image, considered the origin of the image. ArcView will draw the remaining pixels of the image based on this origin.
What is the format of the world file used for georeferencing images?
The world file is an ASCII text file associated with an image and contains the following lines:
Line 1: x-dimension of a pixel in map units
Line 2: rotation parameter
Line 3: rotation parameter
Line 4: NEGATIVE of y-dimension of a pixel in map units
Line 5: x-coordinate of center of upper left pixel
Line 6: y-coordinate of center of upper left pixel
If your image lacks a world file, you can create your own world file, using a text editor. This is generally practical only when the image does not require any rotation or rectification to be properly georeferenced, meaning lines 2 and 3 should be zero. Line 4 is negative to convert from image row numbering (increasing from the top down) to map coordinates (increasing from the bottom up). For a rectified image, line 4 must be equal to line 1 and of opposite sign. The ArcInfo commands REGISTER and RECTIFY, as well as GRIDIMAGE and CONVERTIMAGE create a world file.
The world file must follow this naming convention. If the image file name has a 3-character extension (image1.tif), the world file has the same name followed by an extension containing the first and last letters of the image's extension and ending with a 'w' (image1.tfw). If the extension has more or less than 3 characters, including no extension at all, then the world file name is formed by simply appending a 'w' to the image file name.
The world file is discussed in the ArcInfo Workstation Help documentation under Cartography > Image Integration > Registering an image to real-world coordinates > Georeferencing images.
For additional information on world files, refer to ArcView's online help topic: Help Topics > World files > Georeferenced Images.