World files

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

Some image formats, such as ERDAS, IMAGINE, BSQ, BIL, BIP, GeoTIFF, and grids, store the georeferencing information in the header of the image file. ArcView uses this information if it is present. However, other image formats store this information 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.

World file naming conventions

It's easy to identify the world file which should accompany an image file: world files use the same name as the image, with a "w" appended. For example, the world file for the image file mytown.tif would be called mytown.tifw and the world file for redlands.rlc would be redlands.rlcw. For workspaces that must adhere to the 8.3 naming convention, the first and third characters of the image file’s suffix and a final "w" are used for the world file suffix. Therefore, if mytown.tifmytown.tfw. If redlands.rlc was in an 8.3 format workspace, its world file would be redlands.rcw.

For images that lack an extension, or have an extension that is shorter than three characters, the "w" is added to the end of the file name without altering it. Therefore the world file for the image file terrain would be terrainw; the world file for the image file floorpln.rs would be floorpln.rsw.

How georeferencing information is accessed

The image-to-world transformation is accessed each time an image is displayed (e.g., when you pan or zoom). The transformation is calculated from one of the following sources, listed in order of priority:

the world file
the header file (if the image type supports one)
from the row/column information of the image (an identity transformation)

Because a world file has higher priority, you can override the header file transformation information by creating your own world file.

World file contents

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

	20.17541308822119
	0.00000000000000
	0.00000000000000
	-20.17541308822119
	424178.11472601280548
	4313415.90726399607956

When this file is present, ArcView performs the image-to-world transformation. The image-to-world transformation is a six-parameter affine transformation in the form of:

	x1 = Ax + By + C
	y1 = Dx + Ey + F

where:

x1 = calculated x-coordinate of the pixel on the map
y1 = calculated y-coordinate of the pixel on the map
x = column number of a pixel in the image
y = row number of a pixel in the image
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.

The transformation parameters are stored in the world file in this order:

	A: 20.17541308822119
 	D: 0.00000000000000
	B: 0.00000000000000
	E: -20.17541308822119
	C: 424178.11472601280548
	F: 4313415.90726399607956

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Working Documents last edited on 8 February 2008 at 8:58 am by pool-71-102-172-126.snloca.dsl-w.verizon.net