Acquisition and Standard Corrections
Aster images are acquired from the USGS EROS Data Center (EDC) as raw (1A Level) data. These data are subject to standard radiometric and
geometric correction as documented by the instrument designers. In addition, the Shortwave Infrared (SWIR) channels are processed by a
proprietary radiometric correction that functions as vicarious calibration to surface reflectance. Although it does not remove topographic
effects, the correction allows the SWIR channels to be analyzed directly for mineral absorption features.
The data are cast into a UTM projection WGS84 Datum. The UTM Zone used is based on the longitude of the center of the image. The position
of the image is based on the satellite ephemeris data and is accurate to approximately 200 meters (most images have been found to be accurate
to about 50 meters N-S and 150 meters E-W). The pixels are resampled from the satellite path oriented projection to UTM by cubic convolution.
Mineral Mapping Processing
The technique of mapping individual minerals is based on the fact that Aster SWIR channels are positioned specifically to identify key alteration
minerals. Channels 5 and 6 (2.165 and 2.205 micrometers) are the most critical for high sulfidation alteration as they cover absorptions for
alunite (and pyrophyllite), kaolinite, and dickite.
The goal of the processing is to identify the presence of these minerals as accurately as possible. Unfortunately, since the Aster SWIR channels
have a ground resolution of 30 meters we will rarely see individual pixels that are a pure mineral. In almost all cases, pixels will be a mixture
of a number of minerals.
Because of the mixing problem a procedure is used that involves identifying groups of pixels within an Aster image itself that most closely matches
the target minerals. The signature of these "endmembers" is then used to classify all pixels within the image. The classification technique used
to identify mineral occurrences is called Spectral Correlation Mapping (SCM). Effectively, this technique calculates the mean adjusted 6D angle
between each pixel and the target signature. An value of 1.0 indicates a pixel that is identical in relative signature to the target.
Display
For the purpose of combining as much information as possible into a single image the SCM results are represented as a red, green, blue (RGB)
composite. Alunite (or pyrophyllite) is mapped into red, kaolinite into green and illite into blue. This results in mixtures of colors wherein
bluish pixels are dominated by an illitic signature, greenish a kaolinitic signature and orange-reddish a alunitic signature. Bright colors
represent high concentrations (or confidence) while dark colors represent low concentrations (or confidence).
The results are randomly checked visually against spectral profiles to determine if the classification is reasonably accurate.
Since Aster isn't a hyperspectral instrument, the ability to definitively map minerals is limited. |
