World file

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A world file is a plain text computer data file used by geographic information systems to georeference raster map images. The file specification was introduced by ESRI.[1][2]

Small-scale rectangular raster image maps can have an associated world file for GIS map software which describes the location, scale and rotation of the map. These world files are six-line files with decimal numbers on each line.

Contents

Definition

World files do not specify a coordinate system; this information is generally stored somewhere else in the raster file itself or in another companion file. The generic meaning of world file parameters are:

  • 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[3]
  • 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 rotation parameters are not really rotations (in degree or gradient) and in that as soon as D or B are not zero, the A and E parameter do not correspond to the pixel size anymore. Some people name the D parameter "y skew" and the B parameter "x skew". A better description of the A, D, B and E parameters would be:

  • Line 1: A: x component of the pixel width
  • Line 2: D: y component of the pixel width
  • Line 3: B: x component of the pixel height
  • Line 4: E: y component of the pixel height, almost always negative

All four parameters are expressed in the map units depending on the coordinate system associated with the raster.

When D or B are different than zero the pixel width is given by:

\sqrt{A^2+D^2}

and the pixel height by

\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:

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