Geography Reference
In-Depth Information
the representation and usage of this data poses once
again new challenges. The depth map and grain size map
outputs of the Geosalar project represent over 3 million
measurements spread over an 80 km channel with an
average width of 22m. Visual representations of such a
dataset are problematic. Since the width of the channel is
less than 1% of its length, any scaled map representations
will lose all the detail in the representation. For this
reason, Carbonneau et al. (2012) use a new synoptic,
abstract, representation of river channels where the entire
channel is represented as a rectangular field where widths
are presented in a normalised manner. The main goal of
this visualisation strategy is to achieve a synoptic view of
a given parameter which preserves both downstream and
cross-stream variability. The cross section data are first
reprojected to a constant width of 100 pixels. Experience
has shown that for small rivers, 100 pixels preserves lateral
features (albeit with a loss of scale due to the reprojection).
Second, successive cross-sections are concatenated (i.e.
stacked) in the horizontal direction. This transforms the
river into a rectangular array where each vertical column
represents one cross section and downstream distance
is given horizontally. This abstract river visualisation
allows for the display and viewing of the entire river in
a single compact figure. In essence, this new synoptic
view straightens the channel and widens the width to
allow lateral patterns to become visible. While scaling and
shape are lost due to the strong cross-stream exaggeration
and the elimination of curvature, patterns of variability
in both the cross stream and downstream direction can
now be observed. Figure 9.12 gives an example. On the
left panel, an 11 km stretch of the SMR is represented as
a rectangle where the horizontal scale gives the distance
upstream in kms and the vertical scale gives the channel
widths as a normalised percentage. The colour scheme in
Figure 9.12 is designed to give 'at a glance' information
on habitat suitability. Here the depth and grain size data
were combined to produce a dichromatic range of colours
ranging from yellow to blue. Simply put, yellow areas in
Figure 9.9 represent good juvenile salmon habitat whilst
dark, white and/or blue areas are unsuitable.
9.5.3 Developmentof in-houseairborne
imagingcapabilities
While the airborne optical images of the Geosalar project
were contracted to a private firm, the need to better
control both the cost and parameters of the images
stimulated researchers to develop their own airborne
imaging capabilities.
At INRS, Normand Bergeron's laboratory developed
a helicopter-based imaging system capable of acquiring
high resolution images over hundreds of kilometres of
river (Figure 9.13). The system integrates a FLIR SC660
thermal imaging camera (0.3 megapixels) and Canon
EOS 550D digital SLR (18.7 megapixels) with a precise
hardware triggering system in order to acquire thermal
and optical imagery at 20 cm and 3 cm resolution respec-
tively (from 300m altitude). The two cameras are fixed
to a pan-tilt unit allowing the operator to 'frame' the
image from inside the helicopter, thereby aiding image
acquisition quality. The system is mounted within a heli-
pod, so that it can be used with any suitably licensed and
equipped helicopter operator. The helicopter's GPS posi-
tion and attitude is logged using specialised Matlab code
and stored alongside the images on a laptop computer.
This acquisition system allows imagery to be obtained
rapidly (i.e. 40-50 river kilometres per hour), and at
a fraction of the cost normally associated with similar
remote sensing work.
This platform was recently successfully used to
obtain a continuous coverage of more than 500 km of
100
100
80
60
40
50
20
0
0
0
61
62
63 64 65
Distance upstream [km]
66
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68
69
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71
50
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Depth [cm]
150
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250
Figure 9.12
Dichromatic plot of D50 and depth. This plot represents an 11 km stretch of the St-Marguerite. Here river width is
normalised from 0 to 100% and successive cross sections are stacked horizontally in order to achieve an abstract, synoptic
representation of this entire reach which allows us to conserve lateral variability as well as downstream variability. The colour key on
the right gives the combined values for depth and grain size at any given point.
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