Knowledge Base - Technical Articles
FAQ: What is the format of the world file used for georeferencing images?
| Article ID: | 17489 |
|---|---|
| Software: | ArcInfo Workstation 7.0.4, 7.1.1, 7.1.2, 7.2.1, 8.0.1, 8.0.2, 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3 |
| Platforms: | N/A |
Question
What is the format of the world file used for georeferencing images?
Answer
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.
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.
Created: 3/26/2001
Last Modified: 8/14/2008
Comments
By Anonymous - 07/23/2003 4:00 PM
I do not think that this information is correct. Based on some experimentation I've done with ArcView Image Analysis (which says it supports rotation), the first four tfw file lines are the coefficients in a rotation matrix where the terms are: 1: x pixel size * cos(angle of rotation) 2: y pixel size * sin(angle) 3: x pixel size * sin(angle) 4: -y pixel size * cos(angle) When the angle is zero, this tech article is correct. At other angles, it is not. If you have evidence or document
By Anonymous - 02/23/2005 12:05 PM
How do you find this link? I've searched forever? this is a joke! The world file is discussed in the ArcInfo online documentation under Cartography > Image Integration > Registering an image to real-world coordinates > Georeferencing images.