World file of raster binding

The raster image is stored as a bidimensional file of elements (pixels) where each element has number of a line and a column.

For use of the raster image in GIS the transformation of coordinates of the image pixel (of a line and of a column) to real coordinates on district is necessary.

Parameters of transformation are usually stored with the image.

Some formats of the image, such as GeoTIFF, store the geo-information in the header of a file.

However, other formats of the image (BMP, PCX, TIFF, JPEG and others) store this information in a separate ASCII file.

This file is named as world file. The world file should accompany a file of the image. The world file can be created with use of any text editor.

It has the same name, as a file of the image, with added symbol «w» to extension of a file.

For example, a world file for graphic file RASTER.tiff would be named RASTER.tiffw.

At use of names of files of a format 8.3, extension of a name of a world file is formed differently:

to the first and to the third symbols of extension the symbol «w» is added.

For example: for a file of image RASTER.bmp the world file will be named RASTER.bpW.

World file contains:

- calculated size value of  the image element (pixel) on X axis in units of map (meters);

- 0.0 (value of turn for a line);

- 0.0 (value of turn for a column);

- calculated size value of  the image element (pixel) on Y axis with a minus in units of map (meters);

- x-coordinate of the center of the upper left element of the image (pixel) in units of map (meters);

- y-coordinate of the center of the upper left element of the image (pixel) in units of map (meters).

The size value of the image element is calculated by the formula:

     MeterInElement = Scale / Resolution, where

     Scale - scale of a raster,

     Resolution - permission of a raster at scanning (points on meter).

Example of a world file acapulco.tfw:

1.000000

0.000000

0.000000

-1.000000

402780.914321

1863489.150628

Recently the files of a placing with coordinates in degrees are gaining popularity.

Since the description of a projection parameters in a world file is not stipulated, at loading a raster into a separate window the transformation of coordinates from degrees into meters is carried out by parameters of a sea chart (ellipsoid WGS 84,  the straight cylindrical conformal Mercator projection (Transverse Mercator), rectangular conditional system of coordinates for geographic maps).

At loading the raster image into the open document, the transformation of coordinates is carried out by parameters of the main map of the document.

Thus, knowing onto what ellipsoid the coordinates in degrees for a file of a placing are received, it is necessary to adhere to the scheme of loading for correct positioning a raster.

The scheme of loading of the raster image with use of a world file of a placing in degrees:

1. We create a map or the user map with the necessary parameters of a projection. (item of the menu  File/New/Map or File/New/User map).

2. We open the created map in a separate window.

3. We add into the document the raster image with use of a world file of a placing. (item of the menu File/Add/Raster/*.BMP|*.TIFF|*.JPEG|*.PCX).

Example of a world file Saratov.tfw (coordinates and the sizes of an element are in degrees):    

0.000231146726507495

0.0

0.0

-0.000144010286406934

45.9177874899999949

51.6288470610511894