Applications of Close-range Imagery in River Research - Fluvial Remote Sensing for Science and Management - page 331

vertical imagery is well suited for measuring planform
characteristics, it is less appropriate for quantitative
assessment of vertical features, such as river banks. Sim-
ilarly, features of interest may not be visible from above,
for example as a result of being obscured by a vegetation
canopy. The cost of using an aerial platform can also be
very high because of the cost of acquiring the data and
of getting the platform to the field site. While manned
aerial platforms may be very cost effective for infrequent
surveys covering large areas, the cost may rapidly become
prohibitive for high frequency surveys. Finally, the ability
to secure an airborne platform at short notice may dimin-
ish the ability to document processes that are occurring
with a pseudo random nature. In this context, airborne
platforms become inappropriate for high frequency sys-
tematic monitoring and for spontaneous monitoring in
response to unanticipated events. In river dynamics, short
time scales (hours, days, weeks) are often relevant to doc-
ument and describe significant events like floods that are
rarely predictable, so these arguments are particularly apt
for river geomorphology.
The use of optical imagery from the ground has rapidly
increased over the last few decades as the availability of
affordable instrumentation has improved and because it
offers several key advantages that complement airborne
remote sensing (Lane et al., 1994; 1996; Heritage et al.,
1998; Butler et al., 2001, 2002; Chandler et al., 2002; Car-
bonneau et al., 2003; Graham et al., 2005). In particular,
ground-acquired images have a significant advantage if a
very high temporal or spatial resolution is required (see
Lane et al., 1993). Indeed, ground imagery is the best that
can be achieved in terms of spatial resolution because
sub-centimetre resolutions depend on a limited distance
between the camera and the surveyed object. As sensors
and storage technologies develop, the spatial resolution
obtained from airborne platforms may be expected to
increase. However, where high spatial resolution is more
important than wide areal coverage, surveys conducted
from the ground or low-level unmanned platforms may
be expected to retain an advantage.
In addition, river scientists can benefit from high fre-
quency data capture to investigate rapid phenomena,
like those occurring during individual floods, or to cap-
ture the exact location in space or time where an event
occurs. Close-range image collection is rapid and can
be automated, providing large amounts of information
whilst minimising expensive field time (Lane et al., 1994).
For events of short duration, or where change occurs
rapidly, image-based data capture may be the only prac-
tical means of recording highly relevant information

including time of occurrence or the coincidence of events
and controlling factors. Rapid changes in river morphol-
ogy occur during floods, requiring a survey frequency
that is not feasible with aerial platforms but that is read-
ily feasible with automated ground cameras (Bertoldi
et al., 2009). Because data are collected without physi-
cal contact, they do not result in any disruption to the
objects of the study, making image-based methods ideal in
monitoring studies. Moreover, they may record a wealth
of information beyond that specifically sought for the
project, making them a valuable archive and providing
information that may help to explain behaviours and
patterns that were not anticipated or can otherwise not be
accounted for.
In recent years these factors, allied with rapid advances
in digital imaging technology and parallel decreases in
cost, have led to a widespread adoption of close-range
imagery. This chapter provides an overview of recent
trends in the use of close-range imagery in river research.
First, we describe commonly used technologies and iden-
tify key technical and practical issues for close-range
imagery. Second, we review numerous studies that have
used either vertical or oblique close-range imagery to
document fluvial objects or processes at a wide range of
temporal and spatial scales. Several detailed field examples
are described to illustrate the advantages of close-range
techniques and, more importantly, to emphasise the type
of knowledge that can emerge from their use. Most of the
examples are based on ground acquisition, but one focuses
on helicopter acquisition and combines the advantages of
both airborne and close-range capabilities.

15.2 Technologies and practices

15.2.1 Technology

The photographic technology that is used is not particu-
larly advanced, and commercial standard film or digital
cameras are used, sometimes with a fisheye lens. The tech-
nical specifications of digital cameras are rapidly evolving
so that there is little purpose in trying to describe pos-
sible configurations and technical options. Instead, we
provide three examples of successful setups implemented
in recent studies.
For example, a long term automatic monitoring sys-
tem for the Tagliamento river (Italy) has been installed
on two cliffs overlooking the braidplain in the piedmont
area 2 km and 9 km upstream of Pinzano gorge, respec-
tively (Figure 15.1) (see Bertoldi et al., 2010; Welber et al.,