Landsat 7 ETM+ SLC-Off composite products

In addition to individual SLC-Off products that are each sold separately, ACRES also offers:

1. Landsat 7 SLC-Off ACRES Composite Product - one product made from the combination of several separate scenes (in an attempt to fill the gaps); Ortho-corrected processing only; and
2. Landsat 7 SLC-Off Customer Composite Package - several separate products over the one area for customers to composite in their own way; Path Image processing only. (Recommended for experienced users only.)

To encourage use of Landsat 7 SLC-Off data, both of these options are provided at the same low price.

These composite products and packages provide more useful Landsat 7 data because:

• Customers can choose SLC-Off scenes acquired in the same season (subsequent passes) thereby reducing the ambiguity in interpretation due to temporal changes
• Customers can choose more than two SLC-Off scenes (maximum of five) to provide a greater chance of filling the data gaps
• One additional SLC-On scene can also be included to ensure there are absolutely no gaps in the final product. However this option is less useful for recent acquisitions because an SLC-On scene would now be several years old.

When ordering a Landsat 7 ETM+ SLC-Off ACRES composite product or customer composite package, customers need to nominate a primary SLC-Off scene with up to four other SLC-Off scenes plus additional one SLC-On scene (optional). The aim is for the additional fill scene(s) to fill the gaps as much as possible in the primary SLC-Off scene.

Figure 1. This image illustrates (a) the edge portion of an SLC-Off,
(b) SLC-On and (c) the composite product.

Visual inspection of test products suggests that most land types are restored adequately in the composite products. The image (figure 1) shows an example near the edge portion of a primary SLC-Off scene, an SLC-On scene with equivalent seasonality, and the composite product generated from these two scenes. (Select for higher resolution 175Kb_image)

Figure 2. This image illustrates a composite product generated
from two SLC-Off scenes.

Figure 2 shows another example near the edge portion of a primary SLC-Off scene acquired on 2nd March 2004 (left), an SLC-Off fill scene acquired on 18th March 2004 (middle), and the composite product (right) generated from these two scenes.

For optimal results, the images chosen to generate a gap-filled product should be from the same season and contain minimal transient data such as clouds, snow cover, or fires. Also, customers should avoid selecting fill scenes with quality issues (such as high bit error rate and drop outs which can be identified in ACRES digital catalogue). (Select for higher resolution 96Kb_image)

Details for the ACRES generated SLC-Off Composite Product

The Landsat 7 ETM+ scenes that make up the ACRES composite will each be processed normally through the ACRES satellite image processing system and then combined using the USGS SLC-Off Gap Fill Algorithm. The Landsat 7 ETM+ SLC-Off composite product is generated by replacing the missing data of a primary SLC-Off scene with pixel values derived from one or more co-registered, histogram-matched fill scenes(s).

Figure 3. Histogram matching technique failed due to cloud.

If the images chosen to generate the composite product contain transient data such as clouds, snow cover, or fires, the histogram-matching technique will not work in these areas. The image (figure 3) shows an example near the edge portion of the composite product over the cloudy area. (Select for higher resolution 182Kb_image)

Figure 4. Histogram matching technique failed due to
land/surface change.

Similarly, areas with high temporal variability with respect to land/surface cover will also produce low quality results. The image (figure 4) shows an example near the edge portion of the composite product over cropland. (Select for higher resolution 184Kb_image)

Band-specific gap mask files are included with all SLC-Off composite products processed on or after 04 April 2005. These ancillary data will allow the user to identify the location of all pixels affected by the original data gaps in the primary SLC-off scene and will provide a record of the data source for each pixel. The gap mask files are provided as a series of individual band files, in compressed flat binary format. The product report file will have additional statistic information on the SLC-Off gap fills. For a gap-filled SLC-Off product, the potential data values contained within the gap mask file will be as follows:

Figure 5. This image illustrates the edge portion of three SLC-Off scenes,
the composite product and the gap mask file.

The image (figure 5) shows an example near the edge portion of the composite product generated using one primary and two fill scenes. The gap mask file is shown in colour: Black colour represents no data (DN=0), Blue colour represents primary scene (DN=1), Green colour represents fill scene number one (DN=2), and Red colour represents fill scene number two (DN=3). (Select for higher resolution 149Kb_image)

For detailed information regarding the USGS SLC-Off Gap Fill Algorithm and composite product samples, please visit the links below:

• SLC-Off composite sample data (Off/On and Off/Off) from ACRES FTP site
• ACRES SLC-Off Composite product prices
• USGS LS7 SLC Gap Fill Algorithm [PDF file: 154Kb, Courtesy USGS, NASA]
• SLC-Off usability assessments [Courtesy USGS, NASA]
  Burn Severity Mapping Assessment [PDF file: 2623Kb]
  Canopy Cover/Impervious Surface Assessment [PDF file: 302Kb]
  Crop Type Mapping Assessment [PDF file: 2162Kb]
  Geologic Mapping Assessment [PDF file: 2808Kb]
  Land Cover/Land Use Monitoring [PDF file: 461Kb]