Geography Reference
In-Depth Information
These studies have focused on a variety of topic areas
including channel morphology, grain size and aquatic
habitats (Thorne, 1998; Gordon et al., 2004; Ramakr-
ishna et al., 2011). However, the limitations of traditional
field measurements are well known. Areas of interest
are often considerably larger than can be exhaustively
sampled by field survey, continuous spatial sampling
is generally ideal but rarely feasible at high resolutions
and large extents. Consequently, the scale and resolution
of field-based studies has been constrained by purely
methodological limitations. As argued by Fausch et al.
(2002) this limitation has hampered progress in river
sciences since natural processes often operate on longer
scales both in space and time. Hyperspatial imagery could
make a crucial contribution in this area by allowing
for sampling schemes and experimental designs that can
continuously sample at sub-decimetric resolutions over
reach scales or even over catchment scales. River monitor-
ing studies could also benefit from hyperspatial imagery.
From a purely scientific perspective, many current key
questions concern the evolution of the hydrographic net-
work and the ecology it supports. Experimentally, the
study of these questions requires a network of sampling
sites with paired biological and physical information that
acquire data at a sampling frequency capable of capturing
change at timescales which match the geomorphic and
ecological processes in operation. Such datasets would
also be invaluable in a policy context to river managers
and decision-makers who need feedback on the efficiency
of any restorations actions undertaken. Owing to the
same methodological constraints mentioned above, these
idealised datasets have been nearly impossible to collect.
Hyperspatial imagery is not in itself a solution to the need
for image data sets with a higher temporal resolution.
However, in parallel to the development of hyperspatial
imagery, a number of lightweight and easy to deploy
imaging platforms have become readily available. As a
result, the acquisition of hyperspatial imagery at monthly
or even daily temporal resolutions is now logistically and
economically possible for river reaches of hectametric or
kilometric scales.
In this chapter, we will first summarise the elements of
technical progress in terms of acquisition platform and
camera hardware which are making hyperspatial imagery
a feasible option for scientists and managers. The chapter
then discusses the specific problems and technical issues
that arise when using hyperspatial image data. Finally, we
present case studies which use hyperspatial imagery as a
core data acquisition methodology.
8.2 Hyperspatial image acquisition
When considering an experimental or management
design based on imagery in standard colour format
(RGB), three key ideal image parameters should first be
identified based on the scientific and/or management
requirements of the project:
•
spatial resolution,
•
spatial extent (i.e. size of the study site),
•
temporal resolution.
The successful acquisition of imagery possessing any given
combination of these parameters will be determined by
the following factors:
•
acquisition platform:
◦
ground-tethered devices (e.g. blimps and kites),
Unmanned
Aerial
Vehicles
or
Systems
(UAV
or
◦
UAS),
◦
manned, Ultra Light Aerial Vehicles (ULAV),
◦
general aviation aircraft (e.g. helicopters and Cesna),
◦
presence/absence of infrastructure to enable platform
launch,
acquisition sensor:
◦
•
spatial resolution,
in-flight performance,
◦
logistic and economic factors:
◦
•
total cost,
acquisition conditions and flight execution,
◦
post-processing requirements (staff and software).
◦
8.2.1 Platformconsiderations
The four platform types listed above are those which, to
date, allow for hyperspatial image acquisition. Each of
these platforms has a range of strong and weak points.
Table 8.1 provides an overview and brief literature and
the following sections discuss the characteristics of the
various systems.
8.2.2 Ground-tethereddevices
Ground tethered aerial devices such as kites and blimps
are ideal for applications where it is necessary to monitor
a small area, at either a fixed scale or at multiple scales.
Blimps and kites with sizes from 1m to 2m are gener-
ally capable of lifting loads of 1-3 kg depending on the
exact design. This allows for full size SLR (Single-Lens
Reflex) digital cameras to be carried on specially designed
mounts that maintain the camera in a vertical orientation.
The camera is generally controlled by a radio-remote or
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