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EOM April 2005 Cover Image - Click for Table of Contents

EOM April 2005 > FEATURES

2005 April — Vol. XIV, No. 2

Sam S. Jackson and Scott G. Bourne

Back in May 2003, Landsat 7 developed a technological hiccup with its Scan Line Corrector (SLC). That instrument's job is to compensate for the forward motion of the satellite as its sensor captures data. After several attempts to fix it, scientists determined that the SLC could not be repaired. All data collected since July 2003 (excluding a 2-week interval from 9/3/03 to 9/17/03) have been captured with SLC "off." The result of the failed SLC is Enhanced Thematic Mapper Plus (ETM+) images that have strips of data duplicated and/or missing from the image. The good news is that one part of the scene, the center, is essentially unaffected. The other good news: only about 22 percent of each image is actually lost to the SLC anomaly.

USGS, which runs Landsat on behalf of NASA, has been working for the last several months to provide imagery that will continue to be of maximum value to researchers and others who depend on the data. Last year, USGS introduced three separate product options from the SLC-off data to the market, reflecting three different methods by which the missing data are being handled.

Standard two-pixel interpolation. The uncorrected SLC-off data, Level 0Rp, is "just what the sensor saw" and includes the original duplicated data (Figure 1). Level 1G Standard assigns null values (black) to the areas of duplication, after first applying a small 2 pixel interpolation into the missing data areas (Figure 2).

Gap-filled. Level 1G Gap-filled is just what the name suggests: the data gaps are filled with data from an SLC on scene and/or a series of SLC off scenes over the same geography (Figure 3).

In early gap-filled images, the image requestor selected a single SLC-off image to "fill the gaps." In December 2004, a new choice to select SLC-off images for the "gap- filling" was added. Image requestors now can select several images for the gap filling. That brings the data far closer to an up-to-date image than perhaps any other methodology. Still, it does mean that the requestor of the image needs to research and select the images to use, just as is done with a single SLC-on image.

Level 1G Interpolated. Level 1G Interpolated fills the gaps not with data from other images, but with data interpolated from the same exact SLC-off scene. The interpolation across each gap can run from 2 pixels (which fill only a bit of the gaps) to 15 pixels (which fill the image). The gaps are filled with data interpolated from neighboring scan lines (Figure 4).

The USGS encourages users to learn about the different options to determine which one might be most suitable for the type of work underway. Level 1G standard products provide the closest representation to the actual acquired data, and will contain the original data gaps. Level1G Gap-filled products provide a complete image with no missing data. The gap-filled product, based on data from multiple SLC-on data, gathered at different dates, will produce the best results in geographies with low temporal variability. Level 1G Interpolated products (where the data all comes from the same image date) are useful for large-scale, regional studies which require a fully populated image area and high temporal sensitivity.

The USGS will continue to develop methods to provide the most useful products to the user community.

Figure 1 The edges of Level0Rp data show the duplicate data (left). The center of the scene looks fine (right).

Figure 2 Level1G Standard product includes striped black data gaps at the edges (left), and less striping at the center (right).

Figure 3 The left image is Level 1G Standard collected September 17, 2003; the right is the gap-filled product using previously collected SLC-on data.

Figure 4 Level1G Interpolated data reveal disruptions of features along scan lines at the edges (left), but less pronounced at the center (right).

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