A World file contains geo-referencing data for an image, if present when loading a Background Images it will be read and used to position and scale the image automatically. All World files are assumed to be aligned with the Universal Transverse Mercator (UTM ) coordinate system, therefore rotation is not applied. A World file has the same name as the image but with an extension that can be defined in two ways:
- Either from the first and last character in the images file extension and a 'w'; 'Image.jpg' - World File would be called - 'Image.jgw'
- The same extension with a 'w' on the end; 'Image.jpg' - World File would be called - 'Image.jpgw'
What goes in a World file?
Line 1. x-scale. This is the horizontal distance in meters represented by each pixel in the image.
Line 2. Rotation about y axis. (NOT USED)
Line 3. Rotation about x axis. (NOT USED)
Line 4. y-scale. This is the vertical distance in meters represented by each pixel in the image. Normally negative, because whilst an image has its origin in the top left corner, when dealing with Eastings and Northings, the origin is normally considered to be the bottom left corner, hence the scale is normally negative.
Line 5. x-reference point. This is the horizontal coordinate (or Easting) of the centre of the top left pixel.
Line 6. y-reference point. This is the vertical coordinate (or Northing) of the centre of the top left pixel.
Determining these numbers
The scale is easy to determine if you know the width/height of the drawing:
Image is 4000* 4000 pixels - available via the right-click > Properties of the image in Windows Explorer
And we know the scale of what is shown in the image is 1km * 1km.
X and Y will be the easting and northing of the top left corner of where the image is to be positioned.