World Files

Long ago ESRI programs like ArcView, and MrSid that export TIFF files use World files to register images. This was a simplistic format that should have been retired long ago.  Geotiff is a much better solution because it includes the datum and projection and pixel representation model, and if the only source of georegistration is a world file, you should be dubious about the quality of the data.

The world file has 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.  There is no specification, and these are a poor substitute.

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

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 upper left pixel in meters
F: y Northing UTM coordinate 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.


 The interpretation of the x, y coordinates of the upper left of the image could be either a point in the center of the pixel, or the upper left corner of the pixel.  This depends on the pixel-is-point or pixel-is-area model for the grid. This is usually explicit for a Geotiff, but in keeping with the ad hoc nature of the world file which does not worry about such things as datums, is ambiguous.

If the imagery has a world file, you should pick the world file rather than the image in MICRODEM. The interpretation of MICRODEM for the pixel-is-point or pixel-is-area model  in unclear;  it is at the very bottom of the to-do list, since there is no high quality data being produced using world files.  If you think it is wrong, and you cannot live with the 1/2 pixel offsets, you can manually edit the world files.


Last revised 4/6/2022