Geography Reference
In-Depth Information
the evaluation of impacts of human uses. Characterising
biophysical features continuously along fluvial corridors
is useful for identifying thresholds or discontinuities in
longitudinal trends at a regional scale, and therefore
targeting and planning management actions. Similarly,
when continuous information is not available, a regional
sample of geomorphic features, such as river reaches or
meander bends, can also provide valuable information in
terms of geographical variability. Physical processes are
complex, and a better understanding of different exist-
ing geographical conditions provided by the observation
of a set of contrasting cases is often useful for detect-
ing process-domains and developing testable hypotheses
(e.g. using flume experiments or numerical simulations)
focused on understanding physics and predicting changes
in specific geographical settings.
In this chapter, we aim to summarise how the use
of images can be expanded to relatively large scales of
interest to characterise fluvial corridors, highlight geo-
graphic patterns and develop an understanding of the
spatial and temporal complexity of biophysical processes.
We first detail the raw and sometimes quite heterogenous
remotely-sensed data available for conducting regional-
scale analysis. Then we focus on existing techniques for
analysing longitudinal structures at these scales. Finally,
while much information is available in images, there are
some limitations when using them to cover long, con-
tinuous corridors. We address questions associated with
locating and characterising images, dealing with the res-
olution in relation to the feature size, and addressing the
uneven quality of image mosaics covering large spatial
extents.
most powerful tool for visualising river systems because
of its high resolution (from 15 to 1 meter) and extensive
geographical coverage. Moreover, this platform also pro-
vides historical images. Ten to 15 dates are often available
on river reaches in the USA, much less elsewhere, with
urban areas having the best coverage. Unfortunately, even
though such visualisation platforms integrate external
information which can be superimposed on the images,
the remotely sensed exploitation of the images is still at
an early stage and it is surely here that future innovation
can be expected in terms of both image management and
geoprocessing tool development.
Researchers also have opportunities to access national
imagery resources with better resolution, more homo-
geneous spatial coverage and better documentation
of remote sensing procedures. Systematic acquisition
at a national level began in earnest after the Second
World War, although archived imagery for certain
regions is available as far back as the 1920s. In the
United States, historical imagery is archived at a national
level by the Geological Survey (USGS) at the Earth
Resources Observation and Science (EROS) Center
(http://eros.usgs.gov). Coverage of the entire United
States (excluding portions of Alaska) is available from
the 1980s, with single aerial images and local mosaics
available from about 1940. Notable data sources available
for download at the EROS site include those produced at
1:40,000 scale as part of the National Aerial Photography
Program between 1987 and 2004, the lower-resolution
National High Altitude Photography program between
1980 and 1989, the USGS Digital Orthophoto Quad-
rangle mosaic which covers the entire United States
at approximately 1-m resolution using images dating
from the late 1980s through the 2000s, and sub-meter
resolution Orthoimagery from the early 2000s for major
US metropolitan areas. While many of these data are
available from other sites, the EROS Earth Explorer
webserver (http://edcsns17.cr.usgs.gov/EarthExplorer/)
is helpful because it provides easy access to the
photographic dates available from the EROS archive
for a given locality. Similar data are available for
Canada for a relatively nominal charge from the Natural
Resources Canada National Air Photo Library website
(http://airphotos.nrcan.gc.ca/). Another important
source of more recent US imagery is the National
Agricultural Imagery Program (NAIP), which provides
1-m or 2-m ground resolution orthorectified imagery
for variable portions of the Lower United States each
year starting in 2003. The imagery is generally flown
during the summer growing season. NAIP photographs
11.2 What are the raw data available for
a biophysical characterisation of
fluvial corridors?
The use of air photos for interpreting and characteris-
ing biophysical processes and features is based on a rich
inheritance. Many of the methods introduced here are
simply more automated forms of recognition of spatial
objects that some researchers have long applied on shorter
reaches. But capabilities are now transformed and tremen-
dously increased by the technological changes and the
availability of data.
For the past several years, online images have been avail-
able from websites such as Google Earth, NASA World
Wind, Bhuvan (Indian earth observation visualisation)
and Geoportail in France. Google Earth is presently the
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