World Files

ESRI programs like ArcView, and MrSid export TIFF files use World files to register images. Geotiff is a much better solution because it includes the datum and projection, but world files are with us to stay. 

If the imagery has a world file, you should pick the world file rather than the image.

The world file has two major limitations, which require you to enter data about the projection, which means that it is a poor choice of an incompletely thought out format:

File, Tools Identify directory can create a world file for images of the entire earth.  The upper left corner must be -180, 90 (North Pole, IDL), and the image must have a width twice its height.

File, Save map as image, with world file lets you save the current map as a dumb picture with a world file.


MICRODEM will do its best to figure out the datum and zone.  If there are problems:


Information about building a world file can be found online.

File Type Extension
Any WLD or WORLD
BMP BPW
TIF TFW
JPG JPW or JGW
PNG PNW or PGW
GIF GFW

The world file looks like this, with one value per line:

Line 1

Line 2

Line 3

Line 4

Line 5

Line 6

 

Note: Usually the x value is expressed in meters East, and the y value is expressed in meters North. However, it is possible to work with images and views where the x and y values are expressed in other unit systems, such as decimal degrees (longitude and latitude).

example world file


  • 5.52791260855334
  • 0
  • 0
  • -5.52927276930388
  • 106455.884693458
  • 4025048.87491394

Example of computing a world file, for JPL TM imagery.


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


 

Register images to use in a GIS.

  Meaning Sample
Line 1 dx, decimal degrees (or meters for UTM map) 0.66667
Line 2 0 0
Line 3 0 0
Line 4 dy (negative), decimal degrees (or meters for UTM map) -0.66667
Line 5 x, upper left corner of  image, decimal degrees (or meters for UTM map) -180
Line 6 y, upper left corner of  image, decimal degrees (or meters for UTM map) 90
Sample World File

0.6667

0

0

-0.6667

-180

90


 

 


Last revised 6/29/2016