A2.3 World file - LUPMISManual

A2.3 World File

Level of expertise required for this Chapter: Advanced; general GIS training

A world file is a simple ASCII file with information about the geographical location and resolution of the associated raster file with the same file name.

It contains only six lines:

• Line 1: Pixel size in the x-direction in map units (at UTM: in meters)
  

• Line 2: Rotation about y-axis (at UTM: 0)
  

• Line 3: Rotation about x-axis (at UTM: 0)
  

• Line 4: Pixel size in the y-direction in map units, almost always negative (at UTM: in meters)
  

• Line 5: x-coordinate of the center of the upper left pixel (at UTM: Easting, in meters)
  

• Line 6: y-coordinate of the center of the upper left pixel (at UTM: Northing, in meters)
  

For example:

           0.200000
           0.000000
           0.000000
          -0.200000
      768000.100000
      603999.900000

File sample above indicates, that the raster consists of 20 cm pixels, with center of the upper left pixel at 768000, 604000, no rotation.

The association between raster file and world file is as follows:

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.

World files do not specify a coordinate system, neither they contain datum information. (If the image is in the WGS84 datum, and your other data is in NAD27, there can be a distance discrepancy of hundreds of meters between the depicted position and the true position).

BMP and JPG require a world file to georeference the image for use in many GIS, but not necessarily at Map Maker. TIFF and MRSid, also JPG or GIF for use in Map Maker, can use world files, but can also have the georeferencing information embedded in the image file itself. (For TIFF-format files, these are commonly referred to as GeoTiffs). At ECW files, this is always embedded in the raster file.

For LUPMIS, it is always recommended to have the georeference embedded in the raster file (so-called ‘Map Maker calibration’), to avoid loosing the georeferencing information, which is stored in the separate world file.

Be aware, that editing a raster image in a graphics program will cause georeferencing information be lost, if:

• Georeferencing is embedded (many TIF, all GeoTIF, all ECW, all Map Maker calibrated JPG and GIF)
  

• Resizing
  

• Clipping or Cutting
  

If none of these above, you can edit a raster file, like contrast enhancement, in Paint Shop Pro. If you have a Map Maker calibrated raster file, you have first to create a world file, then do the enhancement, and then re-calibrate it to Map Maker (last step: Main menu > Utilities > Bitmap utilities > Actions > Select file type > Select folder and file > Open > Transform window: Convert format > Convert bitmap from one format to another > OK > Save as window: Select file type > Enter new file name > Save), see also part A of Chapter 5.3.1 for an application example)

For exporting to other GIS installations, which are not running LUPMIS / Map Maker, it is recommended to extract the georeferencing information to a separate world file (see also  Chapter 5.4.4 on transfer of LUPMIS data to other GIS programs).

At Map Maker, there are various tools to transfer world file information (Main menu > Utilities > Bitmap utilities > Actions > Select file type > Select folder and file > Open > Transform window: Convert format and Calibration)