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 graphics software such as 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 and acknowledging sources is an important responsibility for uses in research or publications.
SRTM and ASTER
There are several other web sites to watch. In February 2000 NASA, the US National Image and Mapping Agency (NIMA), and the Italian and German Space Agencies deployed the Shuttle Radar Topographic Mission (SRTM) aboard a Space Shuttle flight. The SRTM uses radar reflection received by two antennae separated by a long arm deployed from the Shuttle to estimate topographic elevation of the Earths surface, with a method known as radar interferometry. The resulting data are in the form of a digital elevation model (DEM) with elevation values for cells 30 metres square. A DEM is therefore a 3-D model of topography, and shows landforms in stunning detail, together with geological features that control them. Because radar relies on energy transmitted from the spacecraft and radar waves can penetrate cloud, the SRTM produces data whatever the time or the weather. The mission successfully captured the entire continental surface between 60°N and 60°S, and will revolutionize both geomorphology and geology. From November 2001 the US Geological Survey and the German Space Agency (DLR) will release DEMs publicly and at low cost, but ‘tasters’ are available from the following web sites: NASA – http://www.jpl.nasa.gov/srtm DLR – http://www.dlr.de/srtm .
For the next decade or so, the main Earth-oriented thrust by NASA is the Earth Observing System (EOS), which will be a constellation of satellites that orbit from pole to pole to give coverage of the entire surface. On 18 December 2000 NASA launched the first of these, named Terra. This satellite carries several payloads that produce images of various kinds, the most geologically important of which is the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), designed and built by the Japanese space agency (ERSDAC), but operated jointly with NASA. ASTER captures images for several ranges of wavelength in the visible, reflected and emitted infrared, which are designed to highlight the spectral properties of common minerals. So, ASTER is a geological remote-sensing system par excellence. The system is working properly, but will begin to produce scientific data sometime in late 2000. Like the SRTM, the plan for ASTER is eventually to produce full coverage of the continental surface during its lifetime. For the moment, you can keep a watching brief by visiting NASA – http://terra.nasa.gov and ERSDAC – http://astweb.ersdac.or.jp