Geography Reference
In-Depth Information
Table 5.1
Comparison of conventional measurements and TIR remote sensing for regional assessment of water temperature in
rivers and streams.
a)
Conventional Measurements
TIR Remote Sensing
•
Measurements can be made at any
point in the water column.
•
Limited technical expertise is
needed to gather data.
•
Data can be obtained under most
weather conditions including fog
and cloud cover.
•
Continuous measurements are
possible using data loggers.
•
Costs of collecting data can be
low, depending on the number of
instruments that must be
deployed.
•
An alternative to collecting validation data is to use existing networks of
in-stream data loggers.
Satellite
•
Capability for regional coverage, repeat monitoring with systematic image
characteristics, and low cost.
•
Data can be gathered across multiple scales from local (e.g. upwelling
ground-water) to regional (entire floodplains).
Airborne
•
Can measure TIR images at fine pixel sizes suitable for narrower streams
and rivers.
Ground
•
Instruments are easy to deploy and validate
in situ
; requires physical access
to the stream.
•
Sparse sampling of
T
k
in space.
•
Gives limited information about
the spatial distribution of water
temperature. Data loggers can be
destroyed or removed by
vandalism or floods.
•
Data are collected only at point
locations. Do not provide a view
the entire thermal landscape of
the river.
•
Temperature gauges are typically
located in larger streams and
rivers.
•
Calibration of thermometers is
still necessary.
•
To collect spatially extensive
measurements, it is necessary to
deploy many personnel.
•
Obtaining TIR images can be costly and complex, and temporally limited.
•
Care must be taken in interpretation of TIR data under off-nadir
observation angles and with variable surface roughness (i.e. diffuse versus
specular reflections).
Satellite
•
TIR images may not be available due to cloud cover, limited duty cycle of
platforms used to collect data (satellite orbits, or availability of aircraft).
Airborne
•
Generally acquired over narrow swath widths covering small areas
compared to satellite data.
•
Acquisition costs can be high, especially if multiple overlapping scan lines
are needed to create a mosaic.
Ground
•
Can only view the water from specific locations along the stream.
•
Observation angles need to be chosen carefully to reduce the effects of
reflections from objects along the river bank.
b)
Conventional Measurements
TIR Remote Sensing
Standard data storage and
processing techniques can be used
(knowledge of the hydrological
system is still necessary).
•
For applications in which having a non-absolute temperature is useful,
non-radiometrically corrected TIR images can be used to assess relative
spatial patterns within a single image.
•
Validation is not required for applications that only need relative
temperatures.
•
•
Interpretation of TIR image data to determine water temperature can be
complex and expensive, and requires trained technical expertise.
•
Care must be taken to interpret TIR images within their terrestrial and
aquatic context.
•
Radiometric correction is necessary to accurately retrieve quantitative
temperatures from TIR data accurately, but this can be time-consuming
and expensive.
For data acquired from aircraft, changes in the stability of the aircraft as it
flies can require complex and costly post-processing of images.
•
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