Long ago ESRI programs like ArcView, and MrSid that export TIFF files use
World files to register images. This was a simplistic format that should have
been retired long ago. Geotiff is a much better solution because it
includes the datum and projection and pixel representation model, and if the
only source of georegistration is a world file, you should be dubious about the
quality of the data.
The world file has major limitations, which require
you to enter data about the projection, which means that it is a poor choice of
an incompletely thought out format:
- No information about the datum used.
- No information about the projection, specifically the UTM zone used, or SPCS zone.
- No information about the pixel-is-point
or pixel-is-area model for the grid. This probably does not make
much difference, since if all you have is data with a world file, the
difference from the pixel model is probably the least cause of concern.
Pixel-is-area might be slightly more common among those who use world files,
but ass-u-me is a possible result of assuming that you know what is not
Identify directory can create a world file for images of the entire earth. The upper left
corner must be -180, 90 (North Pole, IDL), and the image must have a width twice
File, Save map as image,
with world file lets you save the current map as a dumb picture with a world
MICRODEM will do its best to figure out the datum and zone. If there
- Set the correct default UTM zone on the
Datum tab of the options form
- Create an ALL.DATUM file in the directory with the data files, which
will apply to every file in the directory. Copy and rename one of the
datum files that has been correctly created.
- Edit the .DATUM file in the directory with the imagery so that it is
- Use the option, on the
Import/Export tab of the options form,
that will force you to verify the datum and UTM zone every time you open a
world file. (ALL.DATUM present will override this option, so be
careful creating that file).
Information about building a world file can be found
online. There is no specification, and these are a poor substitute.
||WLD or WORLD
||JPW or JGW
||PNW or PGW
The world file looks like this, with one value per line:
- The number of real world units on the x-axis
represented by moving one pixel eastward from any spot (a
- This value is calculated as the x value of the
bottom right-hand corner minus the x value of the
upper left-hand corner divided by the image width in
- The degree to which an image is compressed from the sides
(a negative number)
- This number is usually assumed to be zero. MICRODEM, like many
implementations of world files, will not use this parameter.
- The degree to which an image is compressed from top to
bottom (a positive number)
- This number is usually assumed to be zero. MICRODEM, like
many implementations of world files, will not use this parameter.
- The number of real world units on the y-axis
represented by moving one pixel southward from any spot
(a negative number)
- This number is calculated as the y value of the
bottom right-hand corner minus the y value of the
upper left-hand corner divided by the image height; it
will be negative in almost all cases.
- The x value at the top left of the image
- The y value at the top left of the image
|Note: Usually the x
value is expressed in meters East, and the y value
is expressed in meters North. However, it is possible to
work with images and views where the x and y
values are expressed in other unit systems, such as
decimal degrees (longitude and latitude).
example world file
A: meters/pixel in horizontal x Easting direction
D: rotation about y axis, always 0
B: rotation about x axis, always 0
E: meters/pixel in vertical y Northing direction
C: x Easting UTM coordinate of upper left pixel in meters
F: y Northing UTM coordinate of upper left pixel in meters
These values are used in a six-parameter affine transformation.
x1 = Ax + (By) + C
y1 = (Dx) + Ey + F
x1 = calculated UTM Easting coordinate of the pixel on the map
y1 = calculated UTM Northing coordinate of the pixel on the map
x = column number of a pixel in the image counting from left
y = row number of a pixel in the image counting from top
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
E = negative of y-scale; dimension of a pixel in map units in y direction
Register images to use in a GIS.
- Make a World file for the image. This is a 6 line ASCII
text file with registration information. This is a good job for
NotePad. If you want the 2 km image, you must adjust the dx and dy
- Give the world file the same name as the file you are registering, but give it a
JPW extension, and put it in the same directory.
- The sample below is for a 540x270 pixel image of the entire world.
You must adjust the dx and dy for the image you downloaded. If you
get the 5400 x 2700 images, the spacing is one tenth of the values.
- For a whole world image, dx is 360/(ImageWidth in pixels) and dy is
180/(Image height in pixels)
- Note that you need the NW corner and the pixel spacing.
- The file consists just of the 6 numbers, one per line.
- This will not work with all imagery, since it depends on the map
projection, but will in this case. Because it works, there is severe
distortion at high latitudes, but the whole world is visible.
- After you save the file, verify that NotePad did not add a TXT
extension. If it did, you must rename the file.
- Open the registered image in MICRODEM by selecting the file with the JPW
- Overlay a database to verify your registration.
Koppen climate classification or
Geology options both work well from the Map Window Toolbar
The interpretation of the x, y coordinates of the
upper left of the image could be either a point in the center of the pixel, or
the upper left corner of the pixel. This depends on the
pixel-is-point or pixel-is-area model for the grid.
This is usually explicit for a Geotiff, but in keeping with the ad hoc
nature of the world file which does not worry about such things as datums, is
If the imagery has a world file, you should pick the world file rather than
the image in MICRODEM. The interpretation of MICRODEM for the
pixel-is-point or pixel-is-area model in
unclear; it is at the very bottom of the to-do list, since there is no
high quality data being produced using world files. If you think it is
wrong, and you cannot live with the 1/2 pixel offsets, you can manually edit the
Last revised 4/6/2022