Geography Reference
In-Depth Information
5.4.4 Keypoints
distance between the sensor and the water, and are
typically
<
1m. Many ground-based TIR imaging systems
are available, including forward looking infrared (FLIR)
imagers (Rogalski and Chrzanowski, 2002) that can be
used in both ground-based and airborne surveys. High
quality systems have NE
•
Depending on the TIR sensor and platform that is used,
the available TIR data must be matched to the size of
the river being monitored; as a general rule, when the
channel width is resolved by fewer than three adjacent
pure water pixels in a well-mixed river, the measurements
will have low accuracies and high uncertainties. In most
situations, more than 3 pixels will be needed. Similar
accuracy problems would be expected when resolving
other water bodies.
•
TIR radiation emitted from near-bank objects may
pass directly into the path of the sensor, resulting in
erroneous image interpretation, particularly when the
bank material's temperature is very different from the
water temperature.
•
Having multiple spectral bands is an advantage where
there is a single known temperature, which can be used
as a check of the accuracy of the data processing.
•
Care must be taken to interpret images within their
terrestrial and aquatic context.
•
1
◦
C or better.
When using ground-based TIR imaging sensors, care
must be taken with the viewing geometry of the sensor
relative to the water and the surroundings. In particular,
tall near-bank objects such as trees will scatter emitted
radiation onto the water, and some of this radiation will
be reflected back into the field of view (FOV) of the
imaging sensor (see Section 5.4.3 for more details). Data
from ground-based imaging sensors such as a FLIR can
be calibrated (e.g., Handcock et al., 2006) using shielded
and stirred water targets at different temperatures. Lin-
ear regression is used to relate raw image values (DN)
to T
r
measurements made using a hand-held broad-
band radiometer.
Δ
Tvaluesof0
.
See also Table 5.1.
5.5.2 Airborne imaging
As airborne TIR imaging sensors are widely used for mon-
itoring of water temperature in riverine environments, we
will cover the topic in some detail. Airborne TIR imaging
sensors can be mounted on either fixed-wing aircraft or
on a helicopter which may be manned or unmanned.
The resulting pixel size of the TIR data (instantaneous
field of view, or IFOV) is a function of the distance
from the water surface (determined by the height that the
platform flies), the sensor characteristics, and the optical
FOV of the sensing system. The height of the platform
and optical FOV also determines the ground footprint of
the resulting image.
The narrow swath widths of TIR images from airborne
platforms - typically from a few kilometers for fine reso-
lution images, to a few 10s or 100s of meters for very-fine
resolution images - reduces their ability to capture long
stream reaches as the channel winds in and out of the
preferred straight-line flight pattern. To best manage the
issues of edge pixels (discussed in Section 5.4.2) in air-
borne TIR image acquisition, a balance must be struck
between the resolution of the sensor, the desired resolu-
tion of the image, the width and sinuosity of the stream
channel, and the altitude of the aircraft. Finer pixel sizes
may be obtained from the same sensor when the aircraft is
flown at lower altitudes, however, as the number of times
the aircraft is required to circle around and line-up on a
new straight-line for acquisition of the stream channel,
5.5 TIR imaging sensors
and data sources
In this section, we discuss sources of TIR data, focusing
on image-based sensors rather than point-based sensors
such as radiant thermometers. Point-based sensors are
discussed in the next section in relation to TIR validation.
There are many sensors for measuring TIR data, and their
availability and refinements is changing rapidly. This
chapter does not attempt to provide a comprehensive
overview. Instead, some examples of particular sensors are
used to illustrate the characteristics typical of particular
types of sensors. Common to all sensor platforms is the
issue of calibration. Some TIR imaging sensor systems
have on-board calibration sources, typically a hot and
a cold calibration source. Other systems record only
relative values of emitted radiation, or digital numbers
(DN), andmust be calibrated (see Section 5.6 for methods
of measuring
in situ
water temperature).
5.5.1 Ground imaging
There are many examples of ground-based imaging sys-
tems that provide an array of TIR measurements. When
choosing a TIR imaging sensor, specifications are usu-
ally given as to its pixel size and precision. Pixel sizes
from such imaging systems will be determined by the
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