by Barnaby W. Rockwell¹, Roger N. Clark, K. Eric Livo, Robert R. McDougal, and Raymond F. Kokaly

Open-File Report 02-200
 

2002
 
 
 
 
 

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code.  Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

¹barnabyr@usgs.gov, Denver, Colorado

This report contains information regarding the reflectance calibration of spectroscopic imagery acquired over the Oquirrh and East Tintic Mountains, Utah, by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) sensor on August 5, 1998.  This information was used by the USGS Spectroscopy Laboratory to calibrate the AVIRIS imagery to unitless reflectance prior to spectral analysis.  The Utah AVIRIS data were analyzed as a part of the USEPA-USGS Utah Abandoned Mine Lands Imaging Spectroscopy Project.

The information contained herein pertains to the original reflectance calibration derived solely from the Saltair beach site on the shores of Great Salt Lake.  The reflectance data derived from this calibration becomes markedly affected by residual absorptions due to atmospheric water vapor and carbon dioxide within short horizontal and vertical distances from the calibration site due to the presence of what is believed to be a distinct microclimate by the lake.  Subsequent to the development of this web site, a new reflectance calibration was derived which mitigated these effects.  Reflectance spectra of bright areas of known composition in the East Tintic Mountains, far from Great Salt Lake, were sampled from the calibrated high altitude AVIRIS data cubes and edited, or "polished," to identify artifacts related to residual absorptions of atmospheric gases, particulates, and sensor noise.  The subtle artifacts identified in this way were incorporated into the multiplier spectra derived from the original calibration site, generating new multiplier spectra that were used to re-calibrate the ATREM- and path radiance-corrected cubes to reflectance.  This process generated a reflectance calibration customized for the Oquirrh/East Tintic Mountain region.

• 1.  Using field spectrometer with spectral range and bandpass similar to that of the imaging spectroscopy data being calibrated, characterize a ground calibration site covered by the image data.  An optimal calibration site is large, spectrally bland, and homogenous (e.g. tarps, gravel parking lots, beaches, salt flats, playas, gravel pits, dam faces, intersections of dirt roads, etc.)
• 2.  View field spectra and delete ones with obvious errors.
• 3.  Average remaining field spectra.
• 4.  Use spectral editing program to remove small glitches in average spectrum related to residual atmospheric absorptions and/or sensor noise or artifacts.
• 5.  Multiply glitch-free average spectrum by Spectralon correction spectrum (spectrum of white plate used to calibrate field spectrometer to reflectance units).
• 6.  Convolve result of step #5 to the sampling interval and bandpass of the imaging spectroscopy data being calibrated.
• 7.  PATH RADIANCE CORRECTION.  Locate a dark, shadowed, and vegetated area in your ATREM-corrected image data set (scaled by 20,000) and extract representative spectra.  Average these spectra together.
• 8.  Use spectral editing program to visually estimate a "true" shape for the average of shadowed vegetation pixels generated in step #7.  ATREM over-corrects for path radiance, resulting in a drastic reflectance fall-off below ~0.55 microns (sometimes the fall-off starts at as high as 0.7 microns).  This fall-off must be removed.
• 9.  CALULATE OFFSET SPECTRUM.  Subtract edited, "corrected" spectrum of dark vegetation (result of step #8) from original, un-edited spectrum.
• 10.  Extract spectra of field calibration site from ATREM-corrected image data cube.  Average these spectra together.
• 11.  Subtract OFFSET spectrum (result of step #9) from average image spectrum of calibration site generated in step #10.
• 12.  CALCULATE MULTIPLIER SPECTRUM.  Divide edited field spectrum of calibration site (result of step #6) by the image spectrum of calibration site from which ATREM path radiance overcorrections have been removed (result of step #11).
• 13.  Apply MULTIPLIER and OFFSET to ATREM-corrected imaging spectrometer data (see flowchart):

• 13a.  Apply radiative transfer corrections to the radiance data supplied by JPL by running the ATREM program, using the ATREM command file shown below.
• 13b.  Download the offset and multiplier spectra provided below.
• 13c.  Subtract the offset spectrum from the ATREM-corrected cube.
• 13d.  Multiply the results of Step 13c by the multiplier spectrum.
• 13e.  Multiply the results of Step 13d by a scaling factor of 20,000.  This step allows the reflectance data to be stored with full dynamic range in unsigned integer format (2 bytes per pixel) rather than floating point format (4-bytes per pixel), reducing data storage requirements.

Information contained on this page:

• Photo of Saltair salt flat calibration site
• Average spectrum of calibration site acquired by ASD field spectrometer
• Channel number, wavelength, and FWHM values for 1998 AVIRIS data
• ATREM command file used to apply atmospheric radiative transfer corrections to radiance data
• AVIRIS image of calibration site showing pixels selected for averaging to characterize site
• Ascii list of AVIRIS pixels averaged to characterize calibration site
• ATREM-corrected AVIRIS spectrum of calibration site
• AVIRIS image of dark target showing pixels selected to estimate ATREM path radiance over-correction
• Ascii list of AVIRIS pixels averaged to characterize dark target
• Derived MULTIPLIER spectrum
• Derived OFFSET spectrum

• All pixel locations are given in line, sample (y, x) format where the origin (pixel 1,1) is the upper left pixel of the image.
• Pixel locations apply to the two-scene blocks shown in this image, which also shows the calibration site.

The northern Oquirrh Mountains and the Magna Smelter are visible in the background. The USGS scientists are shown in the process of calibrating the field spectrometer to reflectance units. The Spectralon white plate is held high to reduce the possibility of spectral contamination due to reflections of light from clothing and skin.

Larger 12 KB image

• Click HERE for edited average ASD spectrum of the Saltair beach calibration site in ascii format (shown in green above).

Click HERE for an ascii listing of information regarding 1998 AVIRIS channels. This information comprises the "waves.um.98" file required to run ATREM using the command file shown below. Column descriptions are as follows:
 

Channel Number

Wavelength center  position (microns)

FWHM (microns)

Wavelength uncertainty

FWHM uncertainty

Channel Number

AVIRIS
20.00                           ! plane altitude (in km, above sea level)
08 05 1998 19 12 48             ! date/time (month day year hour minute second)
40 48 04                        ! latitude (degrees minutes seconds)
N                               ! earth hemisphere (N or S)
112 07 31                       ! longitude (degrees minutes seconds)
W                               ! earth hemisphere (E or W)
0.                              ! average spectral resolution (nm) or "0"
waves.um.98                     ! 1998 AVIRIS wavelength file (microns)
1                               ! channel ratio parameters (0 or 1)
0.9140 3 0.9815 3 0.943 3       ! .94 um water vapor band ratio parameters
1.0870 3 1.173 3 1.125 3        ! 1.14 um water vapor band ratio parameters
2                               ! atmospheric model
1 1 1 1 1 1 1                   ! gas selectors
0.34                            ! total column ozone amount (atm-cm)
1 200                           ! aerosol model and visibility (km)
1.90                            ! average elevation of scene surface (in km)
../CUBE/om98_r6_s12_11_rad.v    ! input image cube in radiance units
0                               ! read AVIRIS input data dims from Vicar hdr
../CUBE/om98_r6_s12_11.ATREM3   ! output image cube in scaled reflectance units
0.                              ! spectral resolution (nm) of output data
20000.                          ! scale factor of output reflectance data
om98_r6_s12_11.h2o.vap          ! output water vapor image

Calibration Site: Saltair beach, Great Salt Lake.

The area shown in red corresponds to the area characterized with the field spectrometer.  See below for an ascii list of the pixels shown in red.

Click HERE for an ascii list of coordinates of the AVIRIS pixels averaged to characterize the calibration site. These coordinates are relative to the upper left (NW) corner of AVIRIS scene om98_r6_s12-11.rtgc.

Larger 15 KB image

• Click HERE for an ATREM-corrected average AVIRIS spectrum of the Saltair beach calibration site in ascii format. The spectrum shown in the figure above has had the path radiance corrrection applied. The ASCII version of the spectrum has not had the correction applied.
• The ATREM-corrected spectra of the pixels shown in red above were averaged to produce the spectrum shown here.

• The dark target pixels of shadowed vegetation shown above were selected from AVIRIS scene om98_r6_s10-9.rtgc.
• See below for an ascii list of these pixels.

Click HERE for an ascii list of coordinates of the AVIRIS pixels averaged to characterize the dark target. These coordinates are relative to the upper left (NW) corner of AVIRIS scene om98_r6_s10-9.rtgc.

Larger 17 KB image

• Click HERE for the multiplier spectrum in ascii format.

Larger 14 KB image

• Click HERE for the offset spectrum in ascii format.

References:

Gao, B.C., and Goetz, A.F.H., 1990, Column atmospheric water vapor and vegetation liquid water retrievals from airborne imaging spectrometer data:  J. Geophys Res. 95, pp. 3549-3564.

Gao, B.C., Heidebrecht, K.B., Goetz, A.F.H., ATmospheric REMoval Program (ATREM) User's Guide, 1992, Center for the Study of Earth From Space document, University of Colorado, version 1.1, 24pp.

U.S. Geological Survey, a bureau of the U.S. Department of the Interior
Last modified May 9, 2002.