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:
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.
