International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
1. Introduction
Monitoring, protecting and improving the quality of
waters is critical for targeting conservation efforts and
improving the quality of environment. Methods currently used
to monitor water quality across the landscape consists of in situ
measurements or collection of water samples for analysis in the
laboratory. These techniques, while accurate for a point in time
and space, are time consuming and expensive and do not give
the synoptic views of the landscape necessary to allow
management decisions that can effectively control or improve
water quality.
The Alexandria coastal zone is about 42 km long, extending
from El-Dekhaila in the west to Abu Quir in the east, and
consists of pockets and embayment beaches morphology. In
addition to its moderate temperature in summer and winter, its
beaches, with soft sands and magnificent scenery, are
considered very important natural resources. The coastal zone
of Alexandria is presently experiencing a number of problems
resulting from a considerable amount of wastewater is
discharged into the coastal zone of Alexandria from the
surrounding area as described by (Saad, 1985, Said, 1995 and
Hassan, 1996). This occurs extensively at six regions, Edku lake
inlet, El Tabia pumping station , Eastern Harbor, western harbor
and Mamoura region.
The purpose of the study is to evaluate the potential of using
remotely sensed digital data from Landsat satellite (TM sensor),
to extract information that help in the monitoring system for
Alexandria coastal water quality. The color and surface
temperature information of the coastal water can be derived
from satellite-based observations, as well reflects the main
environmental processes occurring along the coastal water.
These processes can detected through measuring the parameters
that cause changes in the optical characteristics of surface
waters. Each of the components of coastal water contributes to
the values of optical properties for the sea.
The optical properties of sea water are divided into
inherent and apparent properties. The inherent properties are
those associated with the absorption and scattering of light. The
apparent optical properties are those characteristics of the water
body that are dependent on the ambient light, therefore, the
measurements cannot be taken in the laboratory, only in situ.
Apparent optical properties are Secchi disk depth and Irradiance
attenuation. Stramski and Kiefer (1991) and Morel (19912) give
excellent reviews of the optical properties of marine particles.
Observing the marine environment from satellites is a more
recently established method of data capture than aerial
photography, and has undergone a prolific increase in usage
over the last decade. The satellite data are collected in
inherently digital form, and are therefore immediately amenable
to computer processing.
The remote sensing has been started in Egypt since three
decades. The techniques of image processesing were commonly
used for the qualitative studies for the marine and coastal
environment ie. image classification, change detection
techniques, etc.., among these studies are Klemas and Abdel
Kader (1982); Inman and Jenkins (1984); Frihy (1988); Elwany
etal. (1988); Fanos and Khafagy (1989); Ahmed, (1991); Frihy
et al., (1992); Warne and Stanley (1993); El-Raey et al. (1995;
1296
1997, 1998); Yehia, (1998), Ahmed et al., (2000 a,b; 2001;
2002 and 2003).
2. Methodology
In this study, TM landsat images have been used for the
years 1990 and 2000 as shown in figure (1). Landsat sensor
measures radiation in seven bands of the electromagnetic
spectrum with spectral resolution of 30m except for band 6
which measures emitted thermal infra-red radiation and has a
resolution of 120 m. The processing of these color images (two
dates) has been carried out mainly to enhance the water color
and to map the thermal distribution. ERDAS Imagine 8.6
software package is used to process and analyze the acquired
images.
First, The coastal zone of Alexandria is extensively
selected for six regions, based on the existing natural and
human interventions to the coastal water, the six profiles have
been drawn for the six regions representing the clear water (case
| water) or offshore water for profile 1 and case 2 waters or
near shore water for the other five profiles Mamoura region
(profile no 2), El Tabia pumping station (profile no 3), Edku
lake inlet (profile no 4), Eastern Harbor (profile no 5), western
harbor (profile no 6). These profiles are selected based on the
reflectance measurements derived from processing the images.
The location of selected sites is shown in figure (2).
Secondly, as surface water temperature is the basic
parameter for the deviation of the thermal behavior of the
environment, a thermal classification of both images has been
conducted using the thermal infrared (10.4 to 12.5 um) band 6
that measures the amount of infrared radiant flux emitted from
surfaces. The apparent temperature is a function of the
emissivities and true or kinetic temperature of the surface. It is
useful for locating geothermal activity, thermal inertia mapping
for geologic investigations, vegetation classification, vegetation
stress analysis, and soil moisture studies. The resulted
temperature represents an “effective at-satellite temperature of
the viewed Earth-atmosphere system under the assumption of
unity emissivity”. The consideration of emissivity of the surface
cover types would include the additional problem of mixed
pixels in a 60 x 60 m 2 area. Otherwise the mixed signatures
help to accept the assumption of unity emissivity for this kind of
application.
The thermal bands of the satellite images were
transformed into surface temperature values. The digital
numbers were transformed into absolute radiance in the two
landsat sensors (TM5 and TM7), using the following equation :
L = (Lmax - Lmin)/255 * DN + Lmin (1),
where L is the spectral radiance, Lmin and Lmax [mW cm-2 sr-
1 pm-1] are spectral radiances for each band at digital pixel
numbers 0 and 255 respectively.
Using this equation with the TM landsat 5 Lmin and Lmax the
values 0.124 and 1.560 [mW cm-2 sr-1 um-1] respectively.
On the other hand, the using of this equation with the TM
landsat 7 the following reference values are given: ETM +
Spectral Radiance Range:
Low Gain: Lmin - 0.0 Lmax - 17.04 [mW cm-2 sr-1 um-1]
High Gain: Lmin - 3.2 Lmax - 12.65 [mW cm-2 sr-1 pm-1]
FAX ING 4 FM) WOO -— —-
