A world file is a six line plain text sidecar file used by geographic information systems (GIS) to georeference raster map images. The file specification was introduced by Esri.
Filename extension The base filename of a world file matches the raster's base filename, but has a different filename extension (suffix). There are three filename extension naming conventions used for world files, with variable support across software. One simple convention with widespread support is to append the letter "w" to the end of the raster filename. For example, a raster named mymap.jpg should have a world file named mymap.jpgw. An alternative file naming convention that uses a three-character extension to conform to the 8.3 file naming convention uses the first and last characters of the raster file's extension, followed by "w" at the end. For example, here are a few naming conventions for popular raster formats:
A third convention is to use a .wld file extension, irrespective of the type of raster file, as supported by GDAL and QGIS, but not Esri.
Content
The generic meaning of the six parameters in a world file (as defined by Esri) is:
Line 1: A: pixel size in the x-direction in map units/pixel Line 2: D: rotation about y-axis Line 3: B: rotation about x-axis Line 4: E: pixel size in the y-direction in map units, almost always negative Line 5: C: x-coordinate of the center of the upper left pixel Line 6: F: y-coordinate of the center of the upper left pixel This description is however misleading in that the D and B parameters are not angular rotations, and that the A and E parameters do not correspond to the pixel size if D or B are not zero. The A, D, B and E parameters are sometimes named "x-scale", "y-skew", "x-skew" and "y-scale". A better description of the A, D, B and E parameters is:
Line 1: A: x-component of the pixel width (x-scale) Line 2: D: y-component of the pixel width (y-skew) Line 3: B: x-component of the pixel height (x-skew) Line 4: E: y-component of the pixel height (y-scale), typically negative Line 5: C: x-coordinate of the center of the original image's upper left pixel transformed to the map Line 6: F: y-coordinate of the center of the original image's upper left pixel transformed to the map All four parameters are expressed in the map units, which are described by the spatial reference system for the raster. When D or B are non-zero the pixel width is given by:
A 2 + D 2 {\displaystyle {\sqrt {A^{2}+D^{2}}}}
and the pixel height by
B 2 + E 2 {\displaystyle {\sqrt {B^{2}+E^{2}}}}
World files describing a map on the Universal Transverse Mercator coordinate system (UTM) use these conventions:
D and B are usually 0, since the image pixels are usually made to align with the UTM grid C is the UTM easting F is the UTM northing Units are always meters per pixel The above description applies also to a rectangular, non-rotated image which might be, for example, overlaid on an orthogonally projected map. If the world file describes an image that is rotated from the axis of the target projection, however, then A, D, B and E must be derived from the required affine transformation (see below). Specifically, A and E will no longer be the meter/pixel measurement on their respective axes. These values are used in a six-parameter affine transformation:
[ x ′ y ′ ] = [ A B C D E F ] [ x y 1 ] {\displaystyle {\begin{bmatrix}x\prime \\y\prime \end{bmatrix}}={\begin{bmatrix}A&B&C\\D&E&F\end{bmatrix}}{\begin{bmatrix}x\\y\\1\end{bmatrix}}}
which can be written as this set of equations:
x ′ = A x + B y + C y ′ = D x + E y + F {\displaystyle {\begin{aligned}x'&=A\,x+B\,y+C\\y'&=D\,x+E\,y+F\end{aligned}}}
where:
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