Frequently Asked Questions about the Landsat Missions

Question:
How do I fill in the gaps in my SLC-off image?

Answer:

Methods have been developed to better use Landsat 7 ETM+ SLC off data. (For details on Landsat SLC -off, please see http://landsat.usgs.gov/Landsat_7_ETM_SLC_off_data.php.)

A product previously offered by the USGS ‘combined’ two or more Landsat SLC-off scenes that minimized the scan gaps to provide more complete coverage. Now that all Landsat data is available for download at no charge as one standard product, users must now employ other methods to combine images.

Many image-processing packages may have capabilities such as the two described here. (NOTE: References to non-USGS products do not constitute endorsement by the U.S. Government.)

Multiple SLC-off images are required to utilize this method, and all Landsat imagery can be downloaded from EarthExplorer or GloVis. To determine which scenes would be best, please see this FAQ: ‘Which images will work best to fill in the gaps?’

Using ENVI

Preparation:

Individual bands of each image need to be gap-filled before creating a 3-band image. For instance, in order to gap-fill Image1 with Image2, a mosaic will need to be made of Band1 from Image1 and Image2 together. The bands can then be stacked to create the RGB image. (USGS Landsat delivers the bands individually, so simply unzip the files and select the needed bands.)

Gap filling:

Go to Map Mosaicking -> Georeferenced.
Select Import -> Import Files and Edit Properties, and click Open to choose the files you want to gap-fill; they will populate the left-hand frame.
Highlight one file and click OK, setting the Background See Through-Data Value to Ignore to 0.
Color balancing can be done to adjust any brightness differences between the images, if preferred.
Do this for all images.
Click File -> Apply, and assign an output file name and select other applicable options.
Click OK.

Using ERDAS Imagine

Use model-maker to add each band in Image 1 to the corresponding band in Image 2. The following statement says:

Where Image 1 > 0, use Image 1 data, otherwise, use Image 2. Image 2 data will fill the gaps in Image 1.

Use the following syntax (this example is for Band 5) in the model: EITHER $n1_l71015033_03320070515_b50 IF ( $n1_l71015033_03320070515_b50 > 0) OR $n2_l71015033_03320070531_b50 OTHERWISE

Then you can layer stack, or leave the files as individual bands. Note: these directions do not include radiometric matching.

Figure 1. Combining scenes in ERDAS Imagine

While still having a small residual gap, the image below is a combination of the two scenes (without histogram correction applied).

Figure 2. Combined scenes without Histogram Correction in ERDAS Imagine

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