Application Tips for ENVI 5 – Opening and preparing HYPERION imagery for analysis

This is part of a series on tips for getting the most out of your geospatial applications. Check back regularly or follow HySpeed Computing to see the latest examples and demonstrations.

Objective: Unzip, open and save a HYPERION scene, such that the resulting imagery is in a standard ENVI format ready for further processing and analysis.

Scenario: This tip utilizes a HYPERION scene from Kaneohe Bay on the northeast shore of Oahu, Hawaii. This scene was downloaded as a compressed folder from USGS EarthExplorer using the website’s search and browse capabilities to locate and identify a cloud-free scene for Kaneohe Bay. EarthExplorer KaneoheBay The Tip: Below are steps to transform the compressed folder into an ENVI image format:

  • File Formats. HYPERION data is currently available from the USGS in three different product formats: Standard Format L1GST (radiometric corrections and systematic geometric corrections; GeoTIFF), L1R Product (radiometric corrections and pseudo geometric correction using corner points only; HDF) and L1T Product (radiometric corrections and systematic geometric corrections with ground control points; GeoTIFF). Working with the non-geometrically corrected L1R imagery can be useful when implementing pre-processing steps such as destriping. More information on HYPERION and these formats can be found on the EO-1 websites (http://eo1.gsfc.nasa.gov/, http://eo1.usgs.gov/), the EO-1 Data Dictionary (https://lta.cr.usgs.gov/EO1.html) and the EO-1 User Guide (http://eo1.usgs.gov/documents/).
  • Unzip. The first step is to unpack the compressed folder, which in many cases can be accomplished using the tools already available with your operating system. For example, Windows XP/Vista/7/8 allows users to view and copy contents from a .zip archive by double-clicking the folder to open it and then dragging selected files to a new location. Alternatively, if you do not already have such capability, or your file is in a different format (e.g., .tgz), then a good option is to download 7-zip for free from http://www.7-zip.org/.
  • Open Standard Format L1GST. At first glance the files for this GeoTIFF format can appear a bit intimidating since the image data is stored in 242 separate TIFF files. But the scene is actually very easy to open using the associated <*_MTL_L1GST.txt> metadata file included in the same directory. To open the entire scene, including all 242 bands, simply select a standard method for opening files (e.g., File > Open…), and then select the metadata file. This will initiate a dialog that says ‘Opening EO1 Hyperion’. Similar results can be also be achieved by selecting File > Open As > EO1 > GeoTIFF, File > Open As > Generic Formats > TIFF/GeoTIFF, or the ‘Open’ button from the ENVI or Data Manager toolbars.

ENVI Kaneohe Bay

  • Open L1R Product. This product is in HDF format and can be readily opened using File > Open As > EO1 > HDF, and selecting the <*.L1R> file. Note that unlike the GeoTIFF, this product can not be opened using the standard File > Open or ‘Open’ buttons.
  • Open L1T Product. This product is in GeoTIFF format, so it can be opened in the same manner as the Standard Format L1GST, only in this case using the associated <*_MTL_L1T.txt> metadata file.
  • Save as ENVI Format. Now that you have the scene open in ENVI it is straightforward to save it to ENVI format. Simply select File > Save As…, select the relevant file in the ‘Select Input File’ dialog, and then select OK. A ‘Save File As Parameters’ dialog will then appear, in which you select ENVI as the Output Format, select an output filename, and then select OK.
  • Metadata. If you open the associated header file for one of these ENVI files, or examine the metadata using ENVI’s Metadata Viewer, you will see that it contains the wavelength, FWHM, radiance gains and offsets, and solar irradiance defined for each band. Among other uses, these values allow you to calibrate the data to radiance or top-of-atmosphere reflectance using ENVI’s Radiometric Calibration tool.
  • ENVI Header. When examining the file header, you will also note that the resulting files still retain the full original complement of 242 bands; however, only 198 of these bands have been calibrated. A listing of the HYPERION spectral information can be found on the EO-1 website (http://eo1.usgs.gov/sensors/hyperioncoverage). When the bands are ordered sequentially for each detector, the calibrated bands are 8-57 in the VNIR and 77-224 in the SWIR. However, in some situations the resulting header information does not correctly list the proper band designations and other associated spectral information, which can lead to erroneous results. For example, the band located at 851.92 nm is an uncalibrated band and should consist entirely of 1s or 0s. So if your scene indicates otherwise, then you may need to manually edit your header file (the <*.hdr> file is a text file that can be opened and edited in a standard text editor). An example corrected header for the scene from Kaneohe Bay is included here (EO1H0640452010029110K2_L1GST.hdr) as a guideline from which to copy and paste. In all likelihood your scene’s header information for wavelength, fwhm, bbl, data gain values and data offset values should appear similar.
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