Image normalisation: A pre-curser determining the net sediment and
radionuclide fluxes in the Ribble Estuary from airborne remote
sensing.
Richard Wakefield(1), A.N. Tyler(1), P. McDonald(2), & S. Winterbottom(1)
1. Department of Biological and Environmental Sciences, University of Stirling, Stirling. FK9 4LA.
2. Westlakes Research Institute, Westlakes Science and Technology Park, Moor Row, Cumbria CA24 3LN.
Abstract.
The heterogeneous and dynamic nature of estuarine environments renders points
samples spatially and temporally isolated, limiting their ultimate use in estuarine
research. Remote sensing has the potential of placing these measurements into a
quantitative spatial context and provides a valuable data source for quantifying
process and for calibrating and validating hydrodynamic models. Airborne remote
sensing provides the added advantage of enabling multiple scene acquisition over
flood and ebb tide sequences. Such time series airborne remote sensing data is being
used to estimate the net sediment flux in the Ribble Estuary in Lancashire. The strong
association between anthropogenic radionuclide concentrations discharged under
license from the BNFL Sellafield plant provides the added possibility of estimating
the net flux of radionuclides over individual flood and ebb tide sequences (Atkin
2000).
This work presents the methodology and results from a time series investigation of
sediment and radionuclide flux. NERC airborne imagery was acquired in 2003 over
two flood and ebb tide sequences. Imagery from CASI instrument and concomitant
ground reference data collection was used to characterise the suspended sediment
concentration and total load over each scene. The methodology used for image
normalisation is put forward, allowing accurate estimates of suspended sediment
concentrations over a flood and ebb tidal sequence to be generated.
Introduction
The subtidal sediments of the Irish Sea and the intertidal sediments of the Ribble
Estuary represent a substantial repository of artificial radionuclides (McCartney. M.
1994; Beresford Hartwell. P.R. 1995; Mamas. C.J.V. 1995; Assinder. D.J. 1997;
Mudge. S.M. 1997; Kershaw. P.J. 1999; MacKenzie. A.B. 1999; Aldridge 2003).
Gaining quantitative measurements of these intertidal zones has historically been
accomplished by point sampling techniques (Brown. J.E. 1999).
But the
heterogeneous and dynamic nature of estuarine environments renders points samples
spatially and temporally isolated, limiting their ultimate use in estuarine research
(Bryant. R. 1996; Rainey 2003). Remote sensing provides the potential to place these
measurements into a quantitative spatial context and provides a valuable data source
for quantifying process and for calibrating and validating hydrodynamic models.
By obtaining measurements of both the intertidal sediments and the suspended
sediment concentrations by airborne remote sensing it would then possible to see the
interactions of bed sediment characteristics and tidal flow regimes. Rainey et al.,
(2003) demonstrated that it is possible to map the intertidal surface using the
Daedalus 1268 Airborne Thematic Mapper (ATM), allowing the identification of clay
distribution over the intertidal area, which have an intrinsic relationship with
radionuclides in the Irish Sea. This can provide an important tool in understanding
the movement and deposition of heavy metals, such as the anthropogenic
radionuclides discharged under licence by British Nuclear Fuels Limited (BNFL)
Sellafield plant since 1952 (Kershaw. P.J. 1999), which have a high affinity for fine
grained sediments (<2m).
Furthermore, Atkin (2000) demonstrated that it is also possible to map suspended
sediment concentrations over a flood tide with the Compact Airborne Spectrographic
Imager (CASI). This technique allows an estimation of the net flux of sediment that is
transported on the flood tide, but in order to understand the mass balance of sediments
in the estuarine environment, it is also essential to quantify the net sediment flux that
is re-mobilised and transported on the following ebb tide. The asymmetrical nature of
the Ribble Estuary means that the flood tide is shorter than the ebb and because of the
law of mass conversation; the flood velocities become greater than the ebb current
velocities and thus favour an inward transport of sediment (Lumborg 2003) i.e. tidal
pumping. Tidal pumping is counteracted by intermittent high river flow events,
which can effectively erode and transport the tidally pumped sediment back towards
the outer estuary.
The deposition of the fine grained sediments occurs during the slack of high tide and
on the ebb tide, through the interaction of currents, tides and salinity currents (Rainey
2003) and owing to their low shear strength will be rapidly remobilised on the next
tidal cycle. The ability to map the erosion and transportation of these fine-grained
sediments on the flood tide is an important tool in understanding the transportation
pathways and depositional zones ‘sinks’ within the estuarine environment.
But to obtain accurate measurements of suspended sediment concentrations (SSC)
from a set of time series remote sensing images taken over the same flood and ebb
tidal cycle, it is imperative that accurate image normalisation takes place to remove
the effects of changing solar radiance. This can be achieved by a number of
radiometric normalisation techniques (Yuan 1996), of which the empirical linear
spectral normalisation technique was selected.
Methods
Image dataset.
The image data sets were obtained by the Natural Environmental Research Council
ARSF aircraft over the Ribble Estuary (figure 1) on the 17th April 2003 and 15th July
2003. The CASI system on board the aircraft was set to the SeaWIF band selection
that can be used to identify suspended sediment in a marine setting. The solar
radiation is separated into 15 spectral bands, which simulate the satellite-borne
SeaWifs platform. The image data sets consist of 7 west-east orientated flight lines
for April and 12 for July. Concomitant ground reference data collection were used to
characterise the suspended sediment concentration and total load over a number of
images during the July data set, but were not available for the April images.
Radiometric and geometric correction
Radiometric correction was performed externally by NERC ARSF and subsequent
geometric correction was completed using the AZGCORR program supplied by
NERC. This was further enhanced by image to image correction with the RSI ENVI
program, to allow accurate image overlay.
Image normalisation.
The objective of normalisation techniques is to normalise all imagery back to
comparable spectral radiance values for a certain time of the day i.e. the solar zenith.
Image to image normalisation can be conducted either individually by rectifying
image x against a reference y image, so transforming image values in image x to that
of image y. This numerical difference between images can be caused by atmospheric
interference or solar irradiance (Yuan, 1996). Atmospheric (haze) correction requires
ground truth data for all images for a normalisation correction can be carried out. But
Atkin (2000) demonstrated that haze correction was not required when using CASI
band 10 (745-759m). In this study the changing solar irradiance, caused by shifting
solar altitude, appears to be the major explanation for changing radiance values across
the image, accordingly the images were then normalised back to that of the solar
zenith on each particular day. Following on from this linear empirical calibration, it
was then possible to generate maps of ssc over the course of the flood and ebb tidal
cycle. The calibration data was obtained from a past image data series gathered in
1997. This proved necessary due to a lack of ground truth measurements obtained at
time of image acquisition, which will be used as validation points.
Image sub-setting and masking
All non-estuarine areas were masked out to accurately classify the suspended
sediment concentrations in the estuary. This included all exposed intertidal areas and
surrounding salt marsh.
Results
Image normalisation
To normalise the images via changing solar elevation, six points were selected on
Warton aerodrome, located to the north of the image centre. These points were
located upon tarmac slipways that have not been resurfaced within the last 10 years.
They are free from any shadow effect from building and vegetation and run in a
north-south direction. These points are then averaged, to gain their mean change in
radiance value and plotted against time. This is shown in figure 2a/b, where time on
the x-axis represents the time from the image closest to midday (April – 12:21 and
July – 11:54). The y-axis shows the percentage change in the radiance value of the
points over the course of the day.
5
0
-12000
-10000
-8000
-6000
-4000
-2000
0
2000
-5
y = -2E-07x2 + 0.0015x
R2 = 0.9242
% change in radiance
-10
-15
-20
-25
-30
-35
-40
-45
Time (s)
Figure 2a – 17th April 2003: change in radiance values of target areas over time
5
0
-6000
-4000
-2000
0
2000
4000
-5
8000
10000
12000
y = -4E-07x2 + 0.0007x
R2 = 0.8144
-10
% change in Radiance
6000
-15
-20
-25
-30
-35
-40
Time (s)
Figure 2b – 15 July 2003: change in radiance values of target areas over time.
th
Following on from this, the percentage change in radiance is plotted against the solar
elevation angles (obtained from U.S. Astronomic Application Dept., US Naval
Observatory) at time of image acquisition using the regression equation derived from
either figure 3a or 3b. This is then converted into the fractional change value and
applied to each individual image in-turn to normalise them back to the solar zenith for
that particular day
0
0
5
10
15
20
25
30
35
40
45
50
-5
% change in radiance
-10
y = 2.8053x - 133.84
R2 = 0.9052
-15
-20
-25
-30
-35
-40
-45
sun altitude
Figure 3a – percentage change in radiance against solar altitude: Apirl 2003
0
0
10
20
30
40
50
60
70
-5
% change in radiance
-10
-15
-20
y = 2.7224x - 157.8
R2 = 0.8141
-25
-30
-35
-40
Sun altitude
Figure 3b – percentage change in radiance against solar altitude: July 2003
Images.
The sun altitude normalisation technique was then contrasted to the more simplistic
image to image normalisation. Figure 4 shows the comparison between image to
image and sun altitude normalisation for Preston docks which are generated from the
same raw, geocorrected image files. The data, from the July flight lines, shows that
the docks have no interaction with estuarine waters and are not mixed by any currents.
In this way they act as testing areas for the normalisation techniques.
c196051b – 11:48 GMT
c196061b – midday image (11:54 GMT)
c196071b – 13:49
c196081b – 14:02 cloud cover over dock area at time
c196101b – 14:28
Figure 4 – A comparison of image to image normalised images (left) to sun altitude
(right) for flight lines gather on 17th July 2003.
Conclusions/ further work
Sun altitude normalisation appears to produce a more uniform image over the course
of the day, whilst the image to image normalisation generates more variability in the
image output. Normalisation of this type aims to reproduce the radiance value of the
pixel to what it would have been if the image had been obtain at the solar zenith. It is
clear from these images (figure 4) that the sun altitude normalisation generates more
uniform and accurately corrected images.
With image normalisation completed it is then possible to begin to generate estimates
of fluxes of suspended sediment from the whole image. For this calculation,
volumetric estimates must be obtained for the estuary at the times of image
acquisition. This value will be calculated using the Vertical resolving model of
sediment and radionuclide for the Ribble Estuary (VERSE) (Gleizon 1999),
developed by Westlakes Scientific Consulting. It can calculate the total water volume
found within the estuary, applied to which an average of ssc (generated by the remote
sensing imagery) is utilized to estimate the total flux of sediment on the flood tide.
However problems arise on the ebb tide, where the Ribble Estuary becomes stratified,
with a layer of less turbid water overlying a more turbid basal flow that cannot be
observed from remote sensing imagery. This will tackled by using the depth profiles
taken concurrently with the imagery, to estimate net sediment transport on the ebb
tidal sequence. Thus producing an estimate of net sediment flux, which is of great use
in estuarine modelling for estimating dispersement of potential contaminants within
the estuary.
Acknowledgements
The Natural Environmental Research Council for PhD studentship and the remote
sensing facilities provided by the NERC ARSF. Also Westlakes Scientific Consulting
for CASE award and fieldwork support that has made this project possible.
Reference List
1. Aldridge, J. N. Kershaw P. Brown J. McCubbin D. Leonard K. S. & Young E. F. Transport of
plutonium (239/240Pu) and caesium (137Cs) in the Irish Sea: comparison between
observations and the results from sediment and contaminant transport modelling.
Continental Shelf Research. 2003; 23:869-899.
2. Assinder. D.J., Mudge. S. M. & Bourne. G. S. Radiological assessment of the Ribble Estuary - I.
Distribution of radionuclides in surface sediments. Journal of Environmental
Radioactivity. 1997; 36(1):1-19.
3. Atkin. Investigating radionuclide bearing suspended sediment transport mechanisms in the
Ribble Estuary using airborne remote sensing. 2000: Stirling University; 2000p363.
4. Beresford Hartwell. P.R., Horsington. R. W. Mamas. C. J. V. Randle. K. Sokhi. R. S. Taylor. E.
W. & West. J. R. Changes in sedimentation and pollution in the Ribble Estuary. ECSA
20 Symposium : The changing coastline; Hull. 1995; 1995.
5. Brown. J.E., McDonald. P. Parker. A. & Rae. J. E. The vertical distribution of radionuclides in
the Ribble Estuary saltmarsh: transport and deposition of radionuclides. Journal of
Environmental Radioactivity. 1999; 43:259-275.
6. Bryant. R., Tyler. A. N. Gilvear. D. McDonald. P. Teasdale. J. & Ferrier. G. A preliminary
investigation into the spectral characteristics of intertidal estuarine sediments.
International Journal of Remote Sensing. 1996; 17(2):405-412.
7. Gleizon, P (Westlakes Scientific Consulting) 1999. A vertically resolving model of sediment and
radionuclide transport for the Ribble Estuary, M. C. Proceedings of 6th SRP
International Symposium: 291-294.
8. Kershaw. P.J., Dennon. D. C. & Woodhead. D. S. Observations on the redistribution of
plutonium and americium in the Irish Sea sediments, 1978 to 1996: concentrations and
inventories. Journal of Environmental Radioactivity. 1999; 44:191-221.
9. Lumborg, U. & Windelin A. Hydrography and cohesive sediment modelling: application to the
Rome Dyb tidal area. Journal of Marine Systems. 2003; 38:287-303.
10. MacKenzie. A.B., Cook. G. T. & McDonald. P. Radionuclide distribution and particle size
associations in Irish Sea surface sediments: implications for actinide dispersion. Journal
of Environmental Radioactivity. 1999; 44:275-296.
11. Mamas. C.J.V., Earwaker. L. G. & Sokhi. R. S. An estimation of sedimentation rates along the
Ribble Estuary, Lancashire, UK, based on radiocaesium profiles preserved in intertidal
sediments. Environmental International. 1995; 21(2):151-165.
12. McCartney. M., Kershaw. P. J. Woodhead. D. S. & Denoon. D. C. Artificial radionuclides in the
surface sediments of the Irish Sea, 1968-1988. The Science of the Total Environment.
1994; 141:103-138.
13.
Mudge. S.M., Assinder. D. J. & Bourne. G. S. Radioactivity in the Ribble Estuary part 1.
1997.
14. Rainey, M. P. Tyler A. N. Gilvear D. J. Bryant R. G. & McDonald P. Mapping intertidal
estuarine sediment grain size distributions through airborne remote sensing. Remote
Sensing of Environment. 2003; 86:480-490.
15. Yuan, D. & Elvidge C. D. Comparison of relative radiometric normalization techniques. ISPRS
Journal of Photogrammetry & Remote Sensing. 1996; 51:117-126.