You are here: Appendix A - Technical Notes > Notes on World Files

## Notes on World Files

A "world file" is a simple text file containing auxilliary georeferencing information for an image. It can be used to georeference an image that has no georeferencing information within it, or to override existing georeferencing information.

By convention, the filename for a world file is the same as the image it pertains to, with a different extension. The three-letter extension is made up of the first and last characters of the image filename extension, followed by a 'w'. For example, the world file for a TIFF image named bainbridge.tif would be bainbridge.tfw; the world file for a MrSID image named madison.sid would be named madison.sdw.

The LTIGeoCoord class stores the same information as a world file. It has member functions for reading and writing world files. Many of the image reader and writer classes support the use of world file georeferencing information.

###### Format

The world file format is six lines, each line containing a double precision value (represented in text). No additional lines may be present. Leading and trailing whitespace are allowed. The meanings of the six values are:

- dimension of a pixel in map units in x direction
- first rotation term
- second rotation term
- dimension of a pixel in map units in y direction
- x-coordinate of the center of the upper-left pixel
- y-coordinate of the center of the upper-left pixel

The y-dimension is, by convention, a negative value.

The MrSID SDK currently ignores the rotation term in most cases.

Because the floating point values are represented textually, when using world files be wary of errors due to roundoff, imprecise scanf/printf conversion, etc.

###### Example

This is an example of a world file:

xx0.20000000

xx0.00000000

xx0.00000000

x-0.20000000

780.10000000

219.90000000

This world file indicates the image resolution is (0.2, -0.2) and the upper left is at (780.1, 219.9). The rotation terms are zero, meaning no rotation is required.