Tutorial for using the NASA gap filling software

Introduction

This exercise goes through the steps to fill gaps in Landsat ETM+ imagery acquired after the sensor’s scan line corrector (SLC) mechanism failed on May 31, 2003 using freeware that is distributed by NASA. This exercise is designed for use with a Window XP 32-bit computer but the fundamental flow is that same for other operating systems. The NASA gap filling software can be downloaded from the SourceForge web site: [hide]

http://l7gapfill.sourceforge.net

[/hide]. The file to download for a 32-bit Windows computer is: frame_and_fill_win32.zip.

This application works with the standard Level 1 terrain corrected (L1T) GeoTIFF format images distributed by the United States Geological Survey (USGS). These images, as well and the entire Landsat MSS. TM, and ETM+ archive can be ordered and downloaded at no cost through the USGS Global Visualization Viewer (GLOVIS) web site: [hide]

http://glovis.usgs.gov

[/hide]. For more information about the SLC-Off product go to: [hide]

http://landsat.usgs.gov/products_slcoffbackground.php

[/hide].

The processing occurs in two steps. The first step, re-framing, processes all of the input imagery to create images that have the same dimensions in line length and number of lines. The second step uses image data from fill images to replace the no-data (zero-value) pixels in the anchor image (the image to be filled). During the re-framing band 6 data are re-sampled from 60m to 30m resolution. This gap filling process will not fill in all of the gaps but the more fill images that are used the smaller the gaps will be in the gap-filled image.

The processing requires a good deal of disk space since all of the images (anchor and fill) are resized during the re-framing process and a gap filled image is also produced.

Installation

After the “zip” file is downloaded you will need to uncompress it. The open source software 7-Zip ([hide]

http://www.7-zip.org

[/hide]) can be downloaded and installed if other software such as WinZip or WinRAR is not available to you. When you unzip the file a directory labeled “frame_and_fill_win32” will be created which contains all of the files required to run the gap filling process.

Data preparation

Before starting the processing it is necessary to place the anchor and fill images in directories (folders) that are at the same directory level. At this time it is necessary to process Landsat ETM+ bands 1 through 7. The program will stop processing if a subsequent band cannot be found. For example, if band 6 is not included in the directory then the software will process bands 1 through 5 and then the processing with stop. Each of the anchor and fill directories must be labeled using the following naming convention:

anchor = name of the directory holding the bands for the anchor image

fill_scene_1 = name of the directory holding the bands for the first fill image

fill_scene_2 = name of the directory holding the bands for the second fill image

You can have as many fill images as you want as long as you follow the directory naming sequence listed above. The processing priority of the fill images is dictated the directory order. The image in the directory fill_scene_1 will be the first image used to fill gaps in the anchor image. The image in the directory fill_scene_2 (if a second fill image is available) will be used to fill in remaining gaps and so on.

Starting the application

To start the Frame and Fill program you can double-click on the file “frame_and_fill_win32.exe”. This will open a window with 2 buttons at the bottom. Click on the “frame_and_fill_win32” button which will open an IDL welcome screen.

Running the re-frame function

Click on the “RE_FRAME SLC_OFF” button to open the “Reframe_L1T Parameters” window. In the “NUMBER OF FILL SCENES?” text box enter the number of fill scenes that will be used. In the “DIRECTORY PATH TO SCENE FOLDERS?” text box type in the directory path to the directory that contains the anchor and fill directories.

When the number of fill scenes and directory path is entered click on the “SUBMIT” button and processing will start. This step will take several minutes and progress can be monitored in the “Reframe_L1T Parameters” window. When completed the anchor and fill image directories will contain the respective reframed images which have “_reg” added to the end of the file name. For example, if the original file name is: “L71168064_06420070904_B10.TIF” then the file name of the reframed image will be “L71168064_06420070904_B10_reg.TIF”.

A log file showing the details of which images were processed as part of the re-framing is created in the “anchor” directory. An example file name for the log file is: L71168064_06420070904_reframe.txt.

Do not close the “Reframe_L1T Parameters” window if you want to immediately fill the image gaps.

Filling the gaps

Click on the “GAP FILL SLC-OFF” button on the “Frame and Fill” processing window. This will open the “Gap Fill Parameters” window. The number of fill scenes and directory path will be copied from the “Reframe_L1T Parameters” window as long as the “Reframe_L1T Parameters” window was not closed. If this information is not automatically entered you will need to enter it manually using the process mentioned in the previous step (note that a final “\” is needed at the end of the directory path). Click on the buttons labeled with the specific bands you want process or click on “All Bands” to process all of the bands.

Click on the “SUBMIT” button and the gap filling process will begin. This step is quite time consuming and can take a few hours depending on how many fill scenes there are and how fast your computer is. Processing progress is noted at the bottom of the “Gap Fill Parameters” window. When processing is finished the gap filled image bands are created in the “anchor” directory and “_filled” is added to the end of the file name. For example, if the original file name was “L71168064_06420070904_B10.TIF” the name of the gap-filled file will be “L71168064_06420070904_B10_reg_filled.TIF”.

A log file showing the details of which images were processed as part of the gap filling is created in the “anchor” directory. The file name for the gap filling log file is: gapfill_output.txt.

Click on the “DONE” button in the “Frame and Fill” window to close all of the windows.