The preceding article demonstrated how to access and process free Canadian Landsat 7 data from the Canadian Geogratis site. Unfortunately, this site only offers Canadian coverage. However, I recently discovered another website that offers free Landsat TM satellite sensor data from Landsat 4 and/or 5 (I think). This data was apparently purchased from EarthSat Corporation by NASA and is distributed to the public under license. Except for about half of South America and all of Russia, this data set offers comprehensive if not complete world coverage with a friendly user interface that uses the very efficient MrSid data compression algorithms and data structures.

The NASA ESAD web page is one of the strangest remote sensing sites that I have seen. After browsing its pages for an hour I still could not tell for sure what NASA ESAD is and why the page exists. As a result of reading between the lines, I suspect that the site represents some type of initiative by the US government to subsidize the commercial remote sensing industry. It looks as if the directorate is a kind of of centralized purchasing agent for commercial satellite data. This data is then offered to qualified government and academic researchers for free or moderate cost.

One thing is for sure, however. The free satellite data offered at the interactive map section of this site is of high enough quality to be quite useful to the amateur cartographer. In fact, the images are stunning.

To access the data select the ‘Mr. Sid View Imagery of the World’ icon from the Home page. You will be presented with a map of the world showing image coverage. The naming convention of the tiles is as follows: the first component of the name is the hemisphere, the second is the UTM zone, and the third is the latitude of the southern edge of the mosaic. Zoom in on your area of interest by successively clicking on a rectangle. Once you are sure of your location, check the ‘Select EarthSat Mr. Sid Image to View’ radio button and then click on your target quad again. A new window will be generated with a zoomed-out image. Now you must successively adjust the zoom and the resolution until your image is centered at the correct location at the desired resolution. (You will probably want to use the highest resolution setting, which is 1600X1200 pixels.) Once you have generated your target image, download it or copy it by right-clicking you mouse button and then save it in Paintshop Pro. (If you decide to download, beware that the format is .sid. See the previous article on DOQQ processing for information regarding MrSid format.)

The images are obviously false color composites. The information on the NASA ESAD web site that indicated that the images were composed of Band 7 (mid IR), Band 4 (near IR) and Band 2 (visible green). Resolution is 28.5m/pixel. The images cover an area of approximately 170X170km. These are the same statistics described on the EarthSat website for the Individual Landsat TM scenes offered at that site. Judging from the known 47m runway widths at the Boston, MA USA Logan International Airport and Provo, UT USA Municipal Airport visible in the included images this looks about right. Inspection of the image shows that the Logan International runways are faintly visible and just barely resolved. However, the resolution of the Provo, UT USA image appears to be better. The runway widths at Provo Airport are clearly resolved, indicating a resolution definitely <50m. However, the median in Interstate 15 running north/south to the east of Utah Lake is not visible. If I recall correctly, however the median is fairly narrow along the entire stretch from Provo to Salt Lake City.

When the images are saved as a jpeg the RGB channels can be easily separated and recombined but no combination yielded anything close to a true color image. This would apparently require extensive image processing on one or possibly two channels.

I have presented a variety of interesting images to the right. The upper right is of Pearl Harbor, Hawaii USA. The international airport and Ford Island are clearly visible. The second image is of a location in Nepal. The third image is of Boston, MA USA. The fifth image is Provo, UT, USA. Many ground features are clearly recognizable, including of course Utah Lake, Provo Municipal Airport, and the Geneva Steel plant complex. For those that are very familiar with the area, the football stadium at Brigham Young University and University Mall are also visible. The final image is of Provincetown, MA USA, a familiar location to residents of the eastern United States.

Hopefully, this site will continue to offer free data to the public for a while, somewhat compensating for the loss of the free ASTER data mentioned in the previous article. Although nothing yet approaches the resolution and depth of the ASTER data set, this EarthSat data set offers some degree of consolation.

Note: Since I wrote this article, ESAD coverage has expanded to cover the whole earth except for northern Siberia.