• About our digital ortho-photo sources
• What is a world file
• Editing CAPCO ortho-photo world files
• Editing TNRIS 1-meter ortho-photo world files
• Editing TNRIS 2.5 meter ortho-photo world files

The three sources of digital ortho-photos are:

• The City of Austin (for Austin and Travis County)
  The City of Austin has black-and-white orthophotos available for distribution at 2-foot pixel size.  These orthophotos are georeferenced to the Texas State Plane Central Zone, NAD 1983, coordinate system, using  feet as the map units. These orthophotos will come up in the same view as all other City of Austin GIS data sets. No editing of the world file is necessary. You must use the MrSid extension to view these files in ArcView.
   Capital Area Planning Council Austin\Travis County Imagery Web Site
• CAPCO - for the nine county CAPCO region but excluding Travis County
  CAPCO provides web access to black-and-white orthophotos at 1.5 foot pixel size for CAPCO counties outside Travis County. These orthophotos are georeferenced to the Texas State Plane Central Zone, NAD 1983, coordinate system, using meters as the map units. Because the CAPCO data is in meters, the world files have to be edited in order to have them in the same view as all other City of Austin GIS and Round Rock data sets. You must use the MrSid extension to view these files in ArcView.
  Capital Area Planning Council GIS Data Sets Web Site
• TNRIS - for all of central Texas (and eventually all of Texas)
  TNRIS provides access to color-infrared digital orthophoto quarterquads (DOQ's) at 1 meter (on CD-rom) and 2.5 meter pixel resolution (web access). The orthophotos for central Texas are georeferenced to the Universal Transverse Mercator coordinate system Zone 14, NAD 1983, coordinate system, using meters as the map units. The world files need to be edited in order to have them in the same view as all other City of Austin GIS and Round Rock data sets.
  TNRIS DOQ web site

What is a world file?

World files are small text files that can be edited in Wordpad or other text editor. A .tfw file contains 6 lines of text. Lines 1 and 4 express the pixel size in map units (the following .tfw file is the TNRIS DOQ 1-meter expressed in feet in the Texas State Plane Central, NAD 83 coordinate system). Lines 2 and 3 express rotation (in this case 0); lines 5 and 6 represent the coordinates of the center of the upper left pixel on the image. Line 5 is the easting or x coordinate. Line 6 is the northing or y coordinate.

3.28083324
0
0
-3.28083324
3069937.92470864
10155380.50000000

Editing CAPCO orthophoto .tfw files to match City of Austin or City of Round Rock data sets:

The CAPCO ortho-photos are in the same coordinate system as the City of Austin and City of Round Rock data sets, but use meters instead of feet.  Editing the .sdw file does not change the coordinate system unfortunately. However, you should be able to use the MrSid viewer to export to .tif format, and then edit the .tfw file. Every number in the .tfw file needs to be multiplied by the meters to feet conversion factor: 3.28083324

Editing the TNRIS DOQ's (color infrared quarterquads) - 1 meter pixel resolution:

The TNRIS DOQ's are in a different coordinate system and use different map units than the City of Austin and City of Round Rock data sets.  The City of Austin has provided new .tfw files for the 1-meter DOQ's that reference the images to the Texas State Plane Central Zone, NAD83, using feet as map units.  These files are located in the Box-GIS server in Sutton lab under Parmenter\orthophots\doqq-tx-stateplane-83\1-meter_a\ directory.  When you use a  1 meter DOQ  .tif from the TNRIS CD,  do not use the .tfw files that come on the CD. Instead, use the corresponding .tfw from the Box-GIS directory if you want to use the photo in the same view as City of Austin or City of Round Rock data. These files have an "a" at the end of the file name (e.g., 3097341a.tfw)

Editing the TNRIS DOQ's (color infrared quarterquads) - 2.5 meter pixel resolution:

The 2.5 meter pixel resolution DOQ's that you can download from the TNRIS web site are also in UTM Zone 14, NAD 83, and use meters as map units. Again, we have created new .tfw files for these orthophotos (for the Austin area), which you can  also find in the Box-GIS server in Sutton lab under Parmenter\orthophots\doqq-tx-stateplane-83\2.5-meter_b\ directory. When you download a 2.5 meter .tif from TNRIS, use the corresponding .tfw file from the Box-GIS server. These have a "b" at the end of the file name (e.g., 3097341b.tfw). If you look at the new .tfw file, you'll see that we simply took the 1-meter .tfw files from the city of Austin and multiplied lines 1 and 4 (the 1-meter pixel cell size expressed in feet, 3.28083324) by 2.5 to give 8.2020831 feet.