Landsat 7 ETM+ SLC-Off Composite Products

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In addition to individual SLC-Off products sold separately, Geoscience Australia offers:

  • Landsat 7 SLC-Off Composite Product - one product made from the combination of several separate scenes (in an attempt to fill the gaps); Orthocorrected processing only
  • 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 you can choose:

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

When ordering a Landsat 7 ETM+ SLC-Off composite product or customer composite package, you need to nominate a primary SLC-Off scene with up to four other SLC-Off scenes plus one additional 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.

Illustrates (a) the edge portion of an SLC-Off, (b) SLC-On and (c) the composite product. Select for detailed view. 175Kb

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 in Figure 1 shows an example near the edge portion of a primary SLC-Off scene (left), an SLC-On scene with equivalent seasonality (centre) and the composite product (right) generated from these two scenes.

Illustrates a composite product generated from two SLC-Off scenes. Select for detailed view. 96Kb

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 2 March 2004 (left), an SLC-Off fill scene acquired on 18 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 the Geoscience Australia Earth Observation Catalogue.

Details for the Geoscience Australia generated SLC-Off composite product

The Landsat 7 ETM+ scenes which make up the composite will each be processed normally through the satellite image processing system and then combined using the USGS SLC-Off Gap Fill Algorithm [PDF 153KB]. 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).

SLC composite product. Histogram matching technique failed due to cloud. Select for detailed view. 182Kb

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 in Figure 3 shows an example near the edge portion of the composite product over the cloudy area.

SLC composite product. Histogram matching technique failed due to land/surface change. Select for detailed view. 184Kb

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

Similarly, areas with high temporal variability with respect to land/surface cover also will produce low quality results. The image in Figure 4 shows an example near the edge portion of the composite product over cropland.

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 statistical 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:

DN Value Meaning
0 No data
1 Primary SLC-Off scene
2 fill scene #1 (data contributed by fill scene #1)
3 fill scene #2 (data contributed by fill scene #2)
4 fill scene #3 (data contributed by fill scene #3)
5 fill scene #4 (data contributed by fill scene #4)
6 fill scene #5 (data contributed by fill scene #5)

Illustrates the edge portion of three SLC-Off scenes, the composite product and the gap mask file. Select for detailed view. 149Kb

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

The image in 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 represents no data (DN=0), Blue represents primary scene (DN=1), Green represents fill scene number one (DN=2), and Red represents fill scene number two (DN=3).

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

Topic contact: earth.observation@ga.gov.au Last updated: February 10, 2012