World file

A world file is a small text file that contains georeference an image. The file type was introduced by the company ESRI as a supplement for simple image formats. The file name extension is derived from the name and file type of the image file and reads, for example. Jgw. Pgw. Gfw or. Tfw for JPEG, PNG, GIF, or TIFF image data.

File Format

A world file contains 6 rows with the 6 parameters of the affine transformation:

Interpretation of the parameters

The equations of the affine transformation are:

The affine transformation covers not only translation and rotation from the shear, that is often not required. If it is a similarity transformation then: = a11 - a22 a12 = a21 and. Then you can write simplified:

Where:

  • X, y = legal, high value of the position of the pixel.
  • Dx, dy = legal, high value of the pixel size.
  • K, l = column and row of the pixel in the image
  • E, F = easting and northing value of the upper left pixel.

Before dy is a minus, because the y- axis of the image coordinate system faces downward.

In this case, the size of the pixel and the angle of rotation.

If there is no shear to calculate the size of a pixel as follows:

  • A pixel size in the x- direction:
  • Size of a pixel in the y direction:

If the rotation is very small, the parameters a11 and a22 can converge as the size of the pixel will be interpreted in the x and y directions. a21 and a12 are then approximately zero.

In the general case, the distortion present in and shearing in two coordinate systems, the parameters are not to be interpreted graphically.

Due to lack of information regarding the Reference Systems ( projected and geographic coordinate systems ) are world files can only be used in a known context. It is deduced from the length unit and the reference system of the card, in which the image is locates. When image format GeoTIFF these data are binary in the file header.

Example

Example of a world file, matching the average image below:

1.669E -4 0 0 - 9.278E -5 8,491 50 058 Rows 2 and 3, which describe the orientation and linear distortion of the image, are typically specified at 0.0 because a rotation about the respective axes generally omitted. The image should first be corrected accordingly. The units per pixel in the y direction (line 4 ) are usually negative because a picture has its origin at the top left, a geographic coordinate system, however, the bottom left.

The unit of cards works are frequently longitude and latitude. In the example, insert the lines 5 and 6 the upper left coordinate origin at 50.06 ° N and 8.5 ° E finds a place southwest of Frankfurt. Together with the known pixel size of the image file insert the line 1st and 4th image size in degrees on the map.

Analogous to this geographical indication comprises a World file often information that is given in meters per pixel (1st and 4th line ) and to a source of a reference meridian in meters refer (5th and 6th row).

Image rotates inside a rectangle. North is up. World file from the example fits this picture.

Picture a located on a map ( Low distortion due to lack of orthorectification ).

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