World File

Images are stored as raster data, where each cell in the image has a row and column number. In order to display images in real-world coordinates, 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.

This information was stored in a separate ASCII file. This file is generally referred to as the world file, since it contains the real-world transformation information used by the image. World files can be created with any editor. They can also be created using MeteoInfo.

It¡¯s easy to identify the world file which should accompany an image file. The first and third characters of the image file¡¯s suffix and a final "w" are used for the world file suffix. Therefore, for ¡®mytown.tif¡¯, the world file would be ¡®mytown.tfw¡¯. For ¡®redlands.gif¡¯, its world file would be ¡®redlands.gfw¡¯.

The contents of the world file will look something like this:

 

                  20.17541308822119          -A

                   0.00000000000000          -D

                   0.00000000000000          -B

                 -20.17541308822119          -E

              424178.11472601280548          -C

             4313415.90726399607956          -F

 

When this file is present, MeteoInfo performs the image-to-world transformation.

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.


Copyright 2009-2014 Yaqiang Wang. All rights reserved.