tfw files

Most image files are stored in units of inches. However, there are two ways in which an image can know its geographic coordinates. The first option is to imbed special tags in a tif file with the GeoTIFF format. The second option is to have a world file, a 6-line ASCII file file such as

                   2.4384
                   0.0000
                   0.0000
                  -2.4384
              441794.4342
             5094101.4520

These are the 6 coefficients of a three-by-three transformation matrix. They indicate that a pixel is 2.4384 meters square, and that the center of the upper left pixel has an easting (x coordinate) of 441794.4342 and a northing of 5094101.4520. For fans of the matrix:

x = 2.4384 * column + 0.0 * row + 441794.4342
y = (-2.4384) * row + 0.0 * column + 5094101.4520

The second and third values are zero because there is no rotation or skew.

The convention is that a world file is named by jamming a w into the extension. For example, o47122d3.jpg might have a o47122d3.jgw file and o47122d3.tif might have a o47122d3.tfw file. Most of the tif files on the site are geotiff files with redundant tfw files. What happens if the tfw values do not match the geotiff coordinates? In arc/info, the tfw will have precedence. In other software, who knows?

Digression about datums: Most USGS paper maps (and the derived DEMs and DRGs on this site) were done in reference to the North American Datum of 1927. This implies a specific spheroid defining the idealized shape of the earth, as well as reference to the values stamped on brass survey markers. Much newer data, such as the DOQQs served here (well, actually at WAGDA) are referenced to NAD83. This uses a different spheroid, which causes the coordinates of a point (in any coordinate system, including lat-long) to be shifted (at this latitude) about 200 meters north. It also involves resurveying the brass monuments, which causes futher shifts in the UTM coordinates. Changing between NAD27 and NAD83 is extremely complicated, but, for relatively small patches of the earth, it can be closely approximated by shifting the corners of an image.

So what's my point? The DRG images come with redundant tfw files and with tfw83 files. If you replace the tfw file with the tfw83 file, then (IF YOUR SOFTWARE PAYS ATTENTION TO THE tfw FILE), the image will shift about 200 meters to match the NAD83 datum instead of the NAD27 datum. You will have to check to see if it works with your software.

Harvey Greenberg

tfw files