World file description for georeferencing images

File extensions

The extensions of the files that contain data for georeferenciating are obtained through the elimination of the second character from their image file extension and the introducing character w on third position. For example for an image file with extension .tif character "i" is eliminated and character "w" is introduced at the end. In this case the extension for World file will be .tfw. This method is similar for all known image file extensions. To exemplify the extensions of World files are: .bpw, jgw, .pgw, gfw, etc correspondent for .bmp, .jpg, .png, .gif, etc.

More than that it is also accepted extension .wld (that comes from the word World).

Standard format World

The format for standard World file contains 6 lines with the following values:

Line 1: a₁ = pixel size in the x-direction in map units/pixel;

Line 2: b₁ = rotation about y-axis;

Line 3: a₂ = rotation about x-axis;

Line 4: b₂ = pixel size in the y-direction in map units;

Line 5: a₀ = x-coordinate (east) of the center of the upper left pixel;

Line 6: b₀ = y-coordinate (north) of the center of the upper left pixel.

These values are used in a six-parameter affine transformation:

X = a₀ + a₁x + a₂y

Y = b₀ + b₁x + b₂y

The format extended by TopoLT

Besides the standard content of World files TopoLT adds coordinates on the following lines for points that define clipping boundary (ImageClip), in case image has defined a boundary. Coordinates are in image coordinates system (U, V) with the point (0,0) in the center of the pixel (0,0) from the upper-left corner.

If image has defined a rectangular boundary then in the file are written the coordinates for two points, the upper-left and lower-right corners. If polygonal boundary is defined then points coordinates are written, minimum 3 points for polygon.

Figure 1. Image coordinates system for clipping boundary (ImageClip)

Note: The reading of data from file is also made for real values with character comma as decimal symbol.