| 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.