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