|
Two
sources of satellite imagery were used: Landsat
(4) Thematic Mapper (TM) and SPOT (5) Panchromatic
(PAN). Landsat TM was selected as it has seven
multi-spectral bands, thus allowing good land
cover discrimination. Landsat TM has a spatial
resolution of 30 metres, with reflectance values
(stored in separate bands) in the visible, near
infrared, middle infrared, and thermal infrared
spectral regions. A second source of imagery,
SPOT PAN, was acquired to improve the spatial
resolution of the Landsat TM imagery through merging.
SPOT PAN has a spatial resolution of 10 metres
and has a single band, containing reflectance
values in the visible (green) and near infrared
spectrum. Details of the merging technique applied
are provided in Section 4.2.1.
Although
multi-spectral satellite data with a 20 metres
resolution were available (SPOT XS), this imagery
contains data in only three bands and over a narrower
spectrum than Landsat TM. Several studies comparing
classification accuracy of Landsat TM versus SPOT
XS report that Landsat TM consistently outperforms
the other (Green et al 1998; Brockhaus and Khorram
1992).
The
criteria for data selection were that the imagery
had to be cloud free and relatively recent. The
process of selecting suitable imagery was made
more difficult by the fact that:
-
the
swath width of the Landsat TM satellite did
not cover the entire study area in one pass,
which meant that two suitable scenes would
have to be acquired. The Landsat TM satellite
passes over the same location only once every
two weeks;
-
the
dates of the two sources of satellite data
(Landsat TM and SPOT) had to be as near as
possible to ensure that land features/conditions
had not changed;
-
the
SPOT satellite can collect data with and angle
of incidence of up to 278 from the vertical
position, whereas the Landsat TM satellite
can collect data with an angle of incidence
of up to 58 from the vertical position. To
produce a good quality merge, the angle of
incidence of the SPOT image should be close
to that of the Landsat TM image; and
-
Hong
Kong is frequently blanketed by cloud.
Image
data meeting the above criteria were identified
and acquired as described in Table 4.1a
|
Table
4.1a Satellite Imagery Used
|
| Imagery
Type |
Date |
Path/Row |
Time |
Full/Part
Scene |
| Landsat
TM |
26
Sep 1998 |
121/45 |
02:25:10 |
Part |
| Landsat
TM |
3
Oct 1998 |
22/44
and 45 |
02:30:57 |
Part |
| SPOT
PAN |
22
Dec 1998 |
286/305
and 306 |
03:13:32 |
Full |
(4
) The Landsat satellites were launched by the
Aeronautics and Space Administration (NASA) of
USA. The imagery used in this study came from
the Landsat 5 satellite launched on 5 March 1984.
The new Landsat 7 satellite was launched in 15
April 1999, after this study has started.
(5
) The SPOT satellites were launched by Centre
National d'Etudes Spatiales (CNES) of France.
The imagery used in this study came from the SPOT
5 satellite launched on 24 March 1998
|
|