| NOWICKI, Scott A.,
Environmental
Studies, University of Portland, 5000 N Willamette Blvd, Portland, OR
97203
Global
datasets of high-resolution thermal infrared (TIR) and visible to
near-infrared (VNIR) satellite imagery provide opportunities for
mapping planetary surface properties at resolutions that can be
directly applied to field observations, with coverage that allows for
analysis at regional to global scales. This information can be ideal
for investigating geologic processes, climate variables, and surface
history on the terrestrial planets, although advanced processing and
analytical tools are needed to calibrate and display the data in a way
that is useful for field application. To facilitate the use of these
multispectral, multitemporal data in a wider scientific community, a
new data-fusion technique developed using Mars Thermal Emission Imaging
System (THEMIS) multispectral imagery and modified for use with the
Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER)
provides a data product that can be manipulated using commonly
available software to map and interpret surface physical properties
such as sediment grain size, bedrock exposure, and water content of
surface materials.
The term thermal morphology is used here to
describe the combination of daytime visible reflectance with nighttime
brightness temperature, in which the physical properties controlling
the diurnal temperature and albedo can be directly interpreted. Daytime
visible images produce scenes in which the reflectivity, topography,
and surface roughness provide the majority of variation within a field
of view (Figure 1a). Daytime thermal images are similar to daytime
visible, in which the albedo and morphology dominate the temperature
variation within a scene (Figure 1b). Nighttime thermal images display
information related primarily to the thermal inertia of materials, in
which albedo and topographic information is significantly subdued
(Figure 1c). Thermal inertia represents the ability of near-surface
materials to absorb solar energy during the day, conduct it into the
sub-surface, and then release that energy throughout the night. A
combination of these datasets results in a striking image where
colorized nighttime thermal information is draped over daytime data
(Figure 1d).
For application to ASTER, this involves the
compression of co-registered and calibrated 3-band VNIR and 5-band TIR
radiance into a single, RGB-color, byte image (geotiff). In the thermal
infrared emissivity/temperature separation, 5-band nighttime
observations are combined to produce a one-band real number image.
Given the normal range of diurnally-varying temperatures of natural
surfaces on Earth, the observed range can be linearly converted to byte
data range (0-255), and retain 0.1º C temperature resolution.
Daytime
observations are equally compressed in data volume to present the most
thermophysically-significant information in a compressed format. ASTER
3-band VNIR data is integrated and converted to top of atmosphere
calibrated reflectance, producing a single image of minimum data
resolution in albedo of 0.004. Conversion of nighttime temperature to a
standardized hue-based color gradient allows temperature to be draped
over grayscale visible reflectivity resulting in a color image which
displays the information from those two datasets. Retrieval of
temperate and albedo values can be made visually, with the aid of a
color gradient and greyscale. Digital separation of temperature and
albedo can be performed by converting the RGB to HSI (hue, saturation,
intensity) format, in which hue is converted to temperature information
and intensity to albedo. This method allows eight bands of data to be
compressed into a 3-band byte image, and displayed in a format that can
be interpreted in the field while retaining quantitative information
for detailed analysis.
|
