Coastal Baseline Survey of England and Wales Collected by the UK Environment Agency

The Environment Agency is responsible for monitoring the entire
coastal zone as defined by the three nautical mile limit. An
R & D project (Airborne Remote Sensing of Coastal Waters,
published in R & D Report No.4) established the effectiveness of
the combination of aerial remote sensing, continuous underway
monitoring and sampling at selected fixed points.

The image data shows changes in water mass off the survey track
and across the three nautical mile zone.

To be apparent in remotely sensed imagery, changes in water
quality of the coastal zone must in some way alter the
electromagnetic signal reflected or radiated by the water. The
two systems used by the Environment Agency operate in two
different parts of the electromagnetic spectrum, the visible and
thermal bands.

Remotely sensed image data is collected from two sensors, the
Compact Airborne Spectrographic Image (CASI) and the
thermal video scanning system.

THE COMPACT AIRBORNE SPECTROGRAPHIC IMAGER (CASI)

The CASI is an imaging spectrometer designed and built by Itres
Ltd of Calgary, Canada. In use by the Environment Agency, it is
flown over the coastal waters in a leased aircraft.

It works on the principle of a series of lines of charged coupled
devices (CCDs) which each produce an electrical charge
dependant upon the amount of light energy falling upon them.
These are packed together into a 3 dimensional array made up of
288 x 512 CCDs. Upwelling light radiation from beneath the
aircraft is focused onto the array by a compound lens.

Light entering the system is split into a number of wavebands,
each registering on a different CCD or row of CCDs. As the
aircraft travels forward, light reflected from a succession of small
areas fall onto the array and is split into its wavebands. The
minimum size (pixel size) of the areas recorded in the routine
coastal surveys was 8m x 8m. Other pixel sizes can be
recorded by altering height, speed and the focal length of the
lens.

The CASI can be operated continuously in 2 modes and
intermittently in a third. The spatial mode was used in the
coastal surveys. In this mode the CASI records data in up to 19
selected spectral channels from all the pixels across the swath.
Waveband channels appropriate for the observation of
chlorophyll-a, solids in suspension and vegetation in the coastal
fringe were selected and the CASI system was modified by the
addition of a supplementary wide angle lens to cover the three
nautical mile coastal zone in a single pass.

The CASI can also be used in the spectral mode, recording data
from across the whole spectrum over 288 wavebands but only
for a limited number of pixels across the swath. This mode also
allows the collection of spatial mode data in a single waveband
to aid in the location of the spectral data.

Finally, the CASI can also be used in short bursts in an enhanced
spectral mode to record up to 74 channels over a 300 pixel wide
swath. This acts as a middle ground between spectral and spatial
modes.

CASI data from the surveys were processed, enhanced and
interpreted at the National Centre for Instrumentation and
Marine Surveillance using the PCI image processing package
which reads and displays CASI data directly in full 16 bit
resolution to produce high resolution images.

THE THERMAL VIDEO SYSTEM

The thermal imaging system consists of four parts: the scanning
head, the cooling system, the processing electronics/control unit
and VTR/monitor. The system operates in the 8-13 micron
spectral range. The resultant signals are processed to
professional video format with the addition of a data block
containing information on date, time, position and heading.

This system constitutes an inexpensive means of acquiring
thermal data and provides a useful means of identifying the
location and extent of frontal structures, mixing zones and
discharge footprints.

However, it is uncalibrated and can determine only relative and
not absolute temperatures. Also, because it produces analogue
pictorial information, the outputs from the system cannot be
merged with the digital data from the CASI or be geocorrected
to overlay precisely onto maps or other data. The practicalities
of obtaining thermal data in digital form and combining it with
the CASI, GPS and aircraft attitude data are being explored.

GEOMETRIC CORRECTION PROCEDURE

In addition to progressing along their tracks, aircraft move
relative to the ground in pitch and roll motions. Geometric
correction procedures eliminate errors in the images caused by
the movements of the plane and relate the image coordinates to
known geographic coorcdinates. The resultant images can be
overlaid directly onto maps and are of greater use for identifying
the location of features.

Geometric correction of all survey data will allow changes over
time to be accurately mapped and scaled. Images derived from
1995 and future surveys will all be geocorrected.

The geometric correction procedure used by the National Centre
for Instrumentation and Marine Surveillance uses the auxiliary
data collected by the aircraft during the CASI flights. This data
gives information on aircraft motion and geographic location
using a Global Positioning System (GPS).