Another rich source of free GIS data, including Landsat and limited ASTER satellite imagery is the impressive USGS Global Visualization Viewer (GLOVIS) website. This website archives data from several types of satellite sensors. One problem with the site is the diversity of data types. This coupled with the lack of detailed data documentation makes using this site a bit of a challenge to exploit. Be sure to click on the 'Quick Start' and 'Help' buttons at the bottom of the GLOVIS page. These open a series of pages with extensive information on navigating and downloading the data. The thing that is missing and that would be most helpful would be a more detailed description of the data. You can generally research this outside of the GLOVIS site. I have described a few of what I consider to be the more interesting data sets.

Although listed as a data set, no ASTER band data can be downloaded from GLOVIS directly. If you attempt to access ASTER L1A band data for example you will be redirected the ASTER Data Pool for USA coverage (The Data Pool has been described in other articles on this site) or to the ERSDAC site for coverage external to the USA. This latter site appears to be non-functional: at least I have never figured out how to conduct a successful search on it. Global ASTER image data is available from the 'TerraLook' ASTER/Landsat products. TerraLook images are alternatively derived from either Landsat or ASTER bands (you can select either type) that are heavily image-processed to resemble like natural color images by combining spectral bands according to the following recipe:

Red = Red Green = 2/3 Green + 1/3 NIR Blue = 2/3 Green - 1/3 NIR

This makes sense for ASTER since its sensors do not detect in the blue bands. Interestingly, the same formula is used for Landsat even though the latter data set does have a sensor that detects in the visible blue wavelengths. I could not find any direct reference to image resolution but they appear to be the same for both images derived from either sensor: no greater than 30m. The input data appear to have been selected to be mostly cloud-free and the image processing is able to produce some very striking images.

Rather than directly downloading the images you they are sent to you by email. In order to receive this data you will have to establish an account supply some detailed information about yourself on the USGS order form. Although it looks like you might be invoiced, don't worry: the data is free.A couple of examples are shown to the right. The first image is of a remote section of the Chilean coast. The color rendering for this image is excellent considering the lack of a blue sensor. Following it is a corresponding Landsat Terra Look image. Its color fidelity is not as good, although it is also a striking image.

The Landsat 'Legacy' data set is also interesting, in particular the ETM+ and ETM+pan collection. The former is a standard set of ETM band files, including the band 8 panchromatic band. The ETM+pan set includes all the band files in the ETM+ set plus an extra file, a pan-sharpened image at 15m resolution. The sets I downloaded were in.tiff format, which many graphics and GIS applications can open easily. It is possible that some sets are offered only as .L1G files, since this is the other common ETM+ data format. The USGS has done a pretty good job with these. An example of their work can be seen on one of the images to the right.

The GLOVIS website provides a tutorial providing the rudiments of the interface functions, so I will not repeat it here. After designating your target data set, point and click on the global map and you will be zoomed to an area containing a dozen or so path and row data sets. Click on one of them and the metadata table give you useful information about the image, including the percent cloud cover and the date of acquisition. Some collections offer data collected at different times, accessible by clicking on the 'Previous' and 'Next' buttons. These index you through all of the Landsat images that correspond to that path and row number. By browsing all of the available images, you can select the one with the least cloud cover. This also allows researchers who are interested in temporal changes (for example vegetation cover or urban growth) to compare Landsat Images taken over several years. Be sure to explore the menu items at the top of the browse map as they allow you to change collections on the fly, map resolution, and other search variables.

GLOVIS also offers a collection of aerial images. Of course these are only available for the United States but provide convenient reference high-resolution pictures.

GLOVIS is definitely a valuable addition to the archive of free global satellite imagery, its processed ASTER images and Landsat archives being especially useful.