examined thermal-infrared (TIR) image data acquired using the airborne
Advanced Thematic Mapper (ATM) sensor in the afternoon of July 25th, 2000
over a portion of the Colorado River corridor to determine the capability
of these 100-cm resolution data to address some biologic and cultural
resource requirements for GCMRC. The requirements investigated included
the mapping of warm backwaters that may serve as fish habitats and the
detection (and monitoring) of archaeological structures and natural springs
that occur on land. This report reviews the procedure for calibration
of the airborne TIR data to obtain surface water temperatures and shows
the results for various river reaches within the acquired river corridor.
With respect to mapping warm backwater areas, our results show that TIR
data need to be acquired with a gain setting that optimizes the range
of temperatures found within the water to increase sensitivity of the
resulting data to a level of 0.1 °C and to reduce scan-line noise.
Data acquired within a two-hour window around maximum solar heating (1:30
PM) is recommended to provide maximum solar heating of the water and to
minimize cooling effects of late-afternoon shadows. Ground-truth data
within the temperature range of the warm backwaters are necessary for
calibration of the TIR data. The ground-truth data need to be collected
with good locational accuracy. The derived water-temperature data provide
the capability for rapid, wide-area mapping of warm-water fish habitats
using a threshold temperature for such habitats.
daytime TIR data were ineffective in mapping (detecting) both archaeological
structures and natural springs (seeps). The inability of the daytime TIR
data to detect archaeological structures is attributed to the low thermal
sensitivity (0.3 °C) of the collected data. The detection of subtle
thermal differences between geologic materials requires sensitivities
of at least 0.1 °C, which can be obtained by most TIR sensors using
an appropriate gain setting. Simultaneous data collection for both land
and water purposes can be achieved using sensors that collect TIR data
in two separate channels, each channel using a gain setting most appropriate
for land or water. The detection of archaeological structures and natural
water seeps would also be improved by collection of data after sunset,
which would require a separate data acquisition from that providing surface
water temperature data and therefore additional cost.
At this point, the cost for acquiring TIR data is quite high ($620/river-km) compared to the potential benefits of the data, unless reflected-wavelength data are also collected that can satisfy other GCMRC protocol requirements (such as mapping riparian vegetation). This is especially true if multiple data acquisitions are required during the year for temporal analyses of backwater areas. The cost for these data cannot be totally mitigated by its ability to partly replace the need for ground surveys of backwaters because calibration of the TIR data will require some ground-truth data from warm backwater areas (in addition to low-temperature main-stem data). However, the airborne data can provide a product that cannot be approached by ground surveys, that being an instantaneous (2 hour) map of surface water temperature over a 160-km stretch of the Grand Canyon.
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