Geography Reference
In-Depth Information
•
Ground-based TIR imaging sensors are convenient but
can only view the water from specific locations along the
stream. Observation angles need to be chosen carefully to
reduce the effects of scattering from bank objects.
•
Airborne TIR imaging sensors provide spatially exten-
sive images with fine pixel-sizes suitable for narrow
streams and rivers, but acquisition can be costly, pro-
cessing complex, and images are generally acquired over
narrow swath widths.
•
The pixel size and ground footprint of the TIR image is
a function of the altitude above the water body at which
the sensor collects data, the sensor characteristics, and the
optical FOV of the sensing system.
•
A balance must be found between the pixel size of the
TIR sensor and the altitude at which airborne images
are collected. Too high an altitude will result in more
problems with mixed pixels, while too low an altitude will
increase the expense and time required to acquire images
over a given area.
•
Advantages of fixed-wing aircraft include the relative
stability of the platform that simplifies post-processing of
TIR images, and the lower operating costs.
•
Advantages of rotary wing aircraft include the greater
flexibility of the platform for tracking sinuous river-
ine features and for flying at the low altitudes required
for capturing very-fine resolution imagery. Such advan-
tages require additional processing, and can introduce
artifacts into the image which may make image inter-
pretation more complicated, but software advances will
likely reduce these disadvantages in time.
•
UAVs hold great potential for future collection of TIR
imagery, however, the lack of defined operating rules and
licensing procedures for most countries severely limits
their usage.
•
Space-borne TIR imaging sensors provide spatially
extensive images for low cost over large areas, but
pixel-sizes are usually coarser than airborne data and
are commonly too coarse to resolve the river channel,
except for the widest rivers. However their low cost and
capability for regional coverage at multiple points in time
make satellite-based TIR imaging sensors attractive if the
pixel size is suitable for the river size.
•
The typically narrow swath widths of airborne TIR
images make it more likely that overlapping scan lines
must be collected and processed to create a mosaic of
the river.
•
Common to all sensor platforms is the issue of calibra-
tion. Some TIR imaging sensor systems have on-board
calibration sources, while other systems record only rela-
tive values and must be calibrated.
5.6 Validating TIR measurements
of rivers
5.6.1 Timelinessofdata
Water temperature in streams and rivers changes
throughout the day as the sun elevation angle and
heating changes. Collecting validation data for TIR
observations requires that water temperature measured
is close enough to the time of the TIR image collection
that the temperature has not changed significantly. This
can be relatively easy to do in the case of ground-based
images but is much more complex for observations
from aircraft or satellite. In all situations, the collection
of simultaneous measurements can be difficult due to
the requirement for large numbers of personnel. Two
questions that must be addressed pertain to the time of
day of image capture and the duration of the collection
period, both of which depend on the application. For
example, to detect warm groundwater springs during
winter, TIR images should be obtained when the river is
the coldest.
If multiple images are to be compiled into a mosaic
to provide coverage for a larger area, TIR data should be
acquired when water temperature is most stable. River
temperature is typically most stable in the early afternoon
when air temperature is also relatively stable. However,
the thermal inertia of the river water provides additional
stability not found in air temperature, increasing the
time over which river conditions should reflect the ther-
mal conditions during TIR image acquisition. To study
this question more closely, we have provided observa-
tions acquired at an interval of 15min from four sites in
the Pacific Northwest (USA) (Figure 5.9). Two sites are
on a large river (Green River, annual flow 45.6m
3
s
−
1
),
whereas the other two are on smaller streams (Big Soos
Creek, annual flow 3.5m
3
s
−
1
; and a tributary to Cov-
ington Creek, which is a tributary to Big Soos Creek).
Two sites are relatively open with limited riparian cover,
whereas the other two have dense riparian cover (the trib-
utary to Covington Creek is almost completely shaded).
All four sites show a strong diurnal cycle, with mini-
mum temperature occurring just as the sun rises above
the horizon, and maximum temperature occurring by
late morning or early afternoon. Changes in temperature
are typically slowest around these extremes, so that early
morning and early afternoon have the widest sampling
windows with the least change in temperature. The width
of a sampling window is highly dependent on how much
of a change in temperature is acceptable. If observations
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