Geography Reference
In-Depth Information
improved in the last decade (see Chapter 8 and Mar-
cus and Fonstad (2008) for a comprehensive review).
Therefore, publications on the remote sensing of rivers
have dramatically increased and 'Fluvial Remote Sensing'
(FRS) is emerging as a self-contained sub-discipline of
remote sensing and river sciences (Marcus and Fonstad,
2010). Moreover, the technical progress accomplished
in the past two decades of research in FRS means that
this sub-discipline of remote sensing has now begun to
make real contributions to river sciences and manage-
ment and the appearance of a volume on the topic is
therefore timely. Our aim with this edited volume is to
give readers with a minimal background in remote sens-
ing a concise text that will cover the broadest possible
range of potential applications of Fluvial Remote Sensing
and provide contrasted examples to illustrate the capa-
bilities and the variety of techniques and issues. Readers
will notice when consulting the table of contents that we
take a very broad view of 'remote sensing'. In addition
to more conventional remote sensing approaches such
as satellite imagery, air photography and laser scanning,
the volume includes a wider range of applications where
image and/or video data is applied to support river sci-
ence and management. This chapter will set the context
of this volume by first giving a very brief introduction to
remote sensing and by discussing the evolution of journal
publications in fluvial remote sensing approaches and
river management. Finally, we will give a brief outline of
the volume.
such as sonar which use acoustic energy in order to detect
objects in a fluid media such as air or water should be
considered as remote sensing. However it should be noted
that references to remote sensing usually apply to the col-
lection of information via electromagnetic energy such as
visible light, infrared light, active laser pulses, etc. Remote
sensing is then generally divided in two broad categories:
active or passive remote sensing. This description refers
to the source of radiation. Passive remote sensing relies
on externally emitted sources of radiation whilst active
remote sensing relies on internally generated and emit-
ted radiation. The best-known example of active remote
sensing is RADAR (Radio Detection And Ranging) which
uses radio waves to establish the position of objects in
the vicinity of the sensor. More recently, lasers have been
used in active remote sensing to give birth to LiDAR
(Light Detection And Ranging) technology. LiDAR tech-
nology is rapidly becoming the method of choice for the
generation of topography from ground based and air-
borne platforms and is the focus of Chapters 7 and 14 of
this volume.
The key parameter exploited by active remote sensing
has always been the time elapsed between the emission
of a radiation pulse and it's detected return. As a result,
active remote sensing uses a narrow and finite portion
of the electromagnetic spectrum. For example, typical
LiDAR technology uses infrared lasers with a wavelength
of 1024 nm and radar relies on radio waves with wave-
lengths of 1-10 cm. Passive sensors, which rely on an
external source of radiation (usually the sun), make a
much more comprehensive usage of the electromagnetic
spectrum. This is the type of remote sensing which is
familiar to all of us because our visual system uses solar
radiation to detect features in our surroundings. Table 1.1
presents a simplified form of the electromagnetic spec-
trum. This table gives the common names and categories
of radiation as we move, from left to right, from the very
short wavelengths of high energy cosmic radiation to the
very long wavelengths of lower energy micro-waves and
radio waves. Generally speaking, the majority of passive
remote sensing sensor devices applied to earth observa-
tion uses radiation in the visible and infrared portions of
Table 1.1. Given that the electromagnetic spectrum has
a continuous range of frequencies (i.e. radiation wave-
length is not intrinsically discreet), their detection and
quantification relies on sensors that can detect incident
radiation within a specified, finite, range of wavelengths.
The most basic example of this would be greyscale (black
and white) imagery where the brightness of a point on the
photograph is proportional to the total amount of visible
1.2 Remote sensing, river sciences
and management
1.2.1 Keyconcepts inremotesensing
Here we will introduce some key remote sensing concepts
which will help us illustrate and contextualise fluvial
remote sensing as a sub-discipline. However, this intro-
duction is not meant as a foundation text in remote
sensing and we refer the reader in need of some funda-
mental material to classic remote sensing textbooks such
as Lillesand et al. (2008) or Chuvieco and Alfredo (2010).
Remote sensing has amultitude of definitions. In broad
terms, 'remote sensing may be formally defined as the
acquisition of information about the state and condition
of an object through sensors that are not in physical con-
tact with it' (Chuvieco and Alfredo, 2010). This type of
broad definition does not place any restriction on the type
of interactions that occur between the target and the sen-
sor. According to this definition, echo-sounding devices
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