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World FileA 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 six-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. | USGS map of Falkner Island | Example: Original falknermap.jpg is 800600 pixels (detail shown). Its world file is falknermap.jpw and contains: 32.0 0.0 0.0 -32.0 691200.0 4576000.0 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.
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