Some image formats, such as ERDAS, IMAGINE, BSQ,
BIL, BIP, GeoTIFF and grids, store the georeferencing information in the header
of the image file. ArcView uses this information if it is present. The idea is
to write the coordinate information in the world file to "tell" the software
where to the image is located in the real-world.
The world file is a separate ASCII file
containing the real-world transformation
information used by the displaying software.
World files can be created with any editor, such as notepad. They can also
be created using ARC/INFO's REGISTER command.
World file naming conventions
It's easy to identify the world file which should
accompany an image file: world files use the same name as the image, with a "w"
appended. For example, the world file for the image file mytown.tiff would be
called mytown.tiffw and the world file for redlands.rlc would be redlands.rlcw.
For workspaces that must adhere to the 8.3 (xxxxxxxx.yyy) naming convention
(such as DOS, Windows 3.11, and some Win95), the first and third characters of
the image file's suffix and a final "w" are used for the world file suffix.
Therefore, if mytown.tif were in a an 8.3 format workspace, the world file would
be mytown.tfw. If redlands.rlc was in an 8.3 format workspace, its world file
would be redlands.rcw. World files may also use a .wld extension. For images
that lack an extension, or have an extension that is shorter than three
characters, the "w" is added to the end of the file name without altering it.
Therefore the world file for the image file terrain would be terrainw; the world
file for the image file floorpln.rs would be floorpln.rsw.
How the georeferencing information is accessed
The image-to-world transformation is accessed each
time an image is displayed (e.g., when you pan or zoom). The transformation is
calculated from one of the following sources, listed in order of priority:
the world file
the header file (if the image type supports one)
from the row/column information of the image (an
identity transformation)
Because a world file has higher priority, you can
override the header file
transformation information by creating your own
world file.
World file contents
The contents of the world file will look something
like this:
20.17541308822119
0.00000000000000
0.00000000000000
-20.17541308822119
424178.11472601280548
4313415.90726399607956
When this file is present, ArcView performs the
image-to-world
transformation. The image-to-world
transformation is a six-parameter
affine transformation in the form of:
x1 = Ax + By + C
y1 = Dx + Ey + F
where
x1 = calculated x-coordinate of the pixel on
the map
y1 = calculated y-coordinate of the pixel
on the map
x = column number of a pixel in the image
y = row number of a pixel in the image
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
Note The y-scale (E) is negative because
the origins of an image and a
geographic coordinate system are different. 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. Row values in the image
increase from the origin downward, while
y-coordinate values in the map
increase from the origin upward.
The transformation parameters are stored in the
world file in this
order:
20.17541308822119 - A
0.00000000000000 - D
0.00000000000000 - B
-20.17541308822119 - E
424178.11472601280548 - C
4313415.90726399607956 - F
How to build a GeoTIFF header
You need to find the coordinates for the upper
left corner of your image. You can find this information in Idrisi by
displaying
the image and reading the X and Y of the top left corner. The X and the Y are
the C and F of the world file respectively. Pixel size should be the same in
both dimensions and can be read by examing the image description or based upon
the scanning resolution. Then open a text editor and enter the information as
shown at the beginning of the world file contents
section. Save the file with extension as described in the world file
naming conventions section.
