Using satellite images for geological mapping and exploration, or for monitoring short-lived phenomena, such as volcanic eruptions, is now standard Earth sciences technology. But it involves substantial costs for data and for the software needed for analysis, or so it was. Access to the most recent images from the US Landsat-7 and French SPOT systems is now on-line using sophisticated browsing sites on the Web. Both enable guest users, as well as those who have signed up for slightly more sophisticated services, to browse and download reduced-resolution JPEG versions of archived images, and to order data, if needs be. For Landsat-7, go to http://landsat7.usgs.gov/ though this means going through several pages. To jump straight to the Earth Observation System (EOS) Data Gateway try http://edcimswww.cr.usgs.gov:80/ims-bin/pub/nph-ims.cgi?endform=1&u=259015&sid=959259015%2D52891371&mode=SRCHFORM . This currently opens a data search and order form. Choose a search keyword first using the Data Set button, selecting Landsat-7 Level 1 data. You can choose several options for the geographic search area, and simply enter a date range (e.g. 2000-01-01 and 2000-05-25 for this year’s archives. Then Start Search. Sometimes your search will take quite a while, dues to pressure on the server’s bandwidth. The good news is, you can disconnect and go back later to the relevant page using Internet Explorer or Netscape History listing. For SPOT, access is via the DALI server at http://www.spotimage.fr/home/proser/whatdali/daligst/daligst.htm or the Sirius server at http://sirius.spotimage.fr/anglais/Welcome.htm – the Sirius service is a little more complicated than DALI, but is set to become SPOT-Image’s standard browser.
Image quality in both cases is excellent, with the Landsat-7 browse images having a roughly 250 m resolution, and SPOT data showing at about 120 m (4 to 8 times better than similarly available data from meteorological satellites). Use the right mouse button with cursor over the image and select Save Image As: assigning your own name instead of the default given by the server, e.g. geology1.jpg. You can then make some cosmetic changes to contrast and colour balance using MS PhotoEditor or Adobe PhotoShop.
Remember that SPOT data of whatever kind are covered by SPOT-Image copyright, but the USGS who distribute Landsat-7 data make no such claim. Clearing copyright for publication or acknowledging sources is an important responsibility for uses in research or publications.