Encyclopedia > World file

  Article Content

World file

A world file is used by Geographic Information Systems to coordinate raster map images. The standard was created by ESRI corporation of Redlands, California.

Small-scale rectangular raster image maps can have an associated world file for GIS map software which describes the location, scale & rotation of the map.

These world files are 6 line ASCII files with decimal numbers on each line. If the map files end in .jpg or .tif -- then the separate world file ends in .jpw or .tfw for example.

World file Universal Transverse Mercator[?] parameters:

 A: meters/pixel in horizontal x Easting direction 
 D: rotation about y axis, always 0
 B: rotation about x axis, always 0
 E: meters/pixel in vertical y Northing direction
 C: x Easting UTM coordinate of center of upper left pixel in meters
 F: y Northing UTM coordinate of center of upper left pixel in meters

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

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

 x1 = calculated UTM Easting coordinate of the pixel on the map
 y1 = calculated UTM Northing coordinate of the pixel on the map
 x = column number of a pixel in the image counting from left
 y = row number of a pixel in the image counting from top
 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.

Example: Original falknermap.jpg is 800x600 pixels (detail shown). Its world file is falknermap.jpw and contains:


The position of Falkner Island light on the map image is:

 x=171 pixels from left
 y=347 pixels from top

this gives:

 x1=696672 meters Easting
 y1=4565024 meters Northing

Note that the UTM zone is not given so the coordinates are ambiguous -- they can represent a position in any of the 60 UTM zones. In this case, approximate latitude and longitude (41.2, -072.7) were looked up in a gazetteer and the UTM zone was found to be 18 using a web converter.

All Wikipedia text is available under the terms of the GNU Free Documentation License

  Search Encyclopedia

Search over one million articles, find something about almost anything!
  Featured Article

... is still abundant today, herbal medicine[?], miracle cures, and diet and fitness regimes are considered a form of Quackery by medical experts, these criticise the ...

This page was created in 30.9 ms