SHALLOW WATER MAPPING
CORAL
A CASE STUDY FROM THE GREAT
JIL.IIrY. ............... J........
R.E. Reichelt and S.J. Bainbridge
Australian Institute of Marine Science
PMB 3, Townsvnle~ Queensland 4810.
Australia
Commission VII
ABSTRACT
A ground truth study at John Brewer Reef, Great Barrier
(Australia), was carried out
in order to test whether the already established geomorphological mapping of coral reef
zones can be extended to the mapping of the living conlponents of the
substrata.
Present results indicate that the spectral and spatial resolution of Landsat MSS data does
not pelmit the distinction of hard coral substrata from algal substrata, which are the two
dominant classes of living cover. Shallow cora] reef habitats are very heterogeneous over
SPOT XS was
a wide range of spatial scales and therefore the higher spatial resolution
evident in a classified image. A detailed analysis of feature
and separability for
SPOT is in progress. In addition~ the increased spectral resolution of Landsat TM data at
the important blue end of the visible spectrum is presently under
INTRODUCTION
Cora] reefs are often remote and difficult to visit regularly for resource monitoting
purposes. They are usuaHy large, sometimes many kilometres in length, which adds to
the cost of sampling by field observation. The
(OBR) of Australia is
an archipelago of approximately 3000 individual reefs, extending over 2000km. In the last
25 years, many reefs in the central region of the GBR have been altered greatly by
outbreaks of the Crown-of-Thorns starfish, Acanthaster planci,
eats the live coral
allowing subsequent growth of algae on the bare skeletons of the coral. Reefs that have
been severely disturbed by starfish outbreaks have been observed to change from being
coral dominated to algal dominated (Moran, 1986). Existing methods for monitoring the
effects of this starfish all involve underwater sampling by divers which is very costly.
Consequently remotely sensed data are being evaluated as a Olore cost-effective
alternative.
Landsat MSS imagery has already been established as an valuable tool for broadscale
mapping of coral reef zones (Smith~ 1975; Jupp et al. 1985b) at the geomorphological
scale. A reef cover model with classes such as ~reef fronf ~ ~outer reef fIat' and 'lagoon'
was generated by Jupp et al. (1985b) using Landsat data and these classes matched closely
the major geonl0rphological zones for coral reefs defined by Hopley (1982). In order to
study effects of stalfish outbreaks (COT -CCEP Crown-of-Thorns Study, 1986) the
monitoring method must permit the distinction of coral from algae within 3;ny particular
zone and, for ecological monitoring in general, the nlapping of substrata at scales finer
than that of the geonl0rphological zone is required.
Substratum mapping in shallow coral reef areas using satellite inlagery is complicated by
a number of environmental factors. One of the most inlpoliant is the confounding effect
of varying water depth~ both by topographic variation and by tidal variation (Bina and
Ombac, 1979; Jupp et al., 1985b).
Corals and algae are the main living occupiers of space on coral reefs
separability
remotely sensed data is not well known. The brown colour
is detelmined to a large extent
pigments
zooxanthenae~
symbiotic algae
tissue of helmatypic
] 986). An observer in a small boat over a shallow coral reef with relatively
conditions must still actually
in order to assess whether the green-brown
is caused by high algal cover rather than
coral cover. Unfortunately
spectral
signatures of the
groups benthic organisms
not yet been characterised.
by
cover
temporal change in the imagery of a reef, the first step must
to assess the information
content of the imagery at any single point in time. In terms of ground taIth, the main
question in this paper is: Which, if any, of the biological or topographic attributes
measured by ground observers serve to distinguish the various spectral classes detected in
the image data? The substantial histotical archive of Landsat MSS data, and the potential
to compare these historical data with the past records of starfish outbreaks has meant that
the initial emphasis in this study is on the Landsat data.
approximately
region
John Brewer Reef is a crescentic
of the Great Barrier Reef near Townsville. It is 5km on
longest axis and was severely
disturbed by starfish outbreak in 1984-5 (Moran et at., 1985). One Landsat MSS image
and one SPOT in1age of this reef were obtained through the Australian Centre for Remote
Sensing for analysis in this study.
The Landsat MSS (Landsat 5) image was Path 94, Row 73 in Universal Transverse
Mercator Zone 55 with the scene centre at 18°49'S 147°37'E, taken at approximately
0930hrs local tin1e on 20th June 1985. The subset analysed here was 128 pixels x 100
Hnes starting at pixel 2054
line 609 of the image. Local tide height was calculated as
1.4m using a Queensland Department of Harbours and Marine Co-tidal chaI1 for the
Townsville region (Drawing no. 01-155Sh2).
The SPOT XS (multispectral) image (processed to level 1b) was from scene K,L =
373,387 with a subset of 512 pixels x 480 lines,
at pixel 600 in line 2000. The
scene was recorded at approximately 1030hrs local time on 24th July 1987.
tide
height was calculated as 1. 2m, Unfortunately this image was acquired in Dec 1987, after
the field work had been completed and a detailed analysis of the spectral data is still in
progress.
After subsetting the images from computer-compatible tape on a VAX, all image analysis
was done using a microBRIAN system (Jupp et aI., 1985a). Both images were contrast
enhanced and then spatially rectified using 20 control points located on an orthophoto
map of John Brewer Reef prepared by the Australian Survey Office (now caned the
Surveying and Land Information Group, Canberra). During rectification the images were
Locating a
both resampled onto a common
grid producing 5 pixels x 400
pixel that represents a pal1icular ground control point in an image of a reef is sometimes
difficult because there are no regular features and most are not sharply defined.
Using a supervised, nearest-neighbour "" ....'vv~,.u. (Jupp et al. ~ 1985a) the Landsat data were
into 25 themes. The supervision of the
classified into 17 themes. and the
image (raw data minus mean values of each
classification involved
use
a
regions of the image that have radiance values
class - Jupp and Mayo. 1982) to
widely separated from the means the existing classes. These regions were then selected
an
ainled at reducing the infornlation
as new seeds for
content
vV.!,'-'U'U-l
classes
were
on
by surveyors who
(April 1987). The
2.
File
:AJBREW
SensorzLands~t MSS
Date I 28-86-85 Bands '4-S,S-G#6=R
H~me
Im~Qe
1.
Location :John Brewer Reef
Processlng ' Rectlfled Raw Im~ge
showing sample site locations.
Each of these ten sites were sampled by a SCUBA diver using 4 contiguous benthic Hne
transects (Reichelt et
. 1986) each 50m in length with substratum and topographic
vadation being recorded at every metre. Topographic conlplexity along the transects was
vertical relief. to the nearest 10cm~ fronl one metre to the next.
recorded as fluctuation
Details of the sanlpling techniques are given in Bainbridge and Reiche1t( 1988). The sites
vaIied in depth from 1m to 3m.
505 000
File Na.e :ASBREW
SensarlSPOr XS
Image Date I 24-07-87 Bands 11~BJ2.G,3.R
Location IJohn Brewer Reer
Processlng ' Rectlfied Raw Image
]
AINS/COTSAC
image of John Brewer Reef showing structure of reef flat habitats
sand channels.
et al. (l
or color.
(MA)
ems above the
coral from those with
to
its greatly enhanced spatial
. led to a more complex
Landsat MSS were
MA
1
13
5
3
0
0
2
0
3
6
21
1
1
6
7
1
2
3
I
4
5
II
II
1
50
3
5
6
7
8
I
9
17
0
]0
10
5
10
5
5
5
5
10
31
31
27
3]
60
35
36
I
34
23
3
300
1
100
40
100
60
60
30
16
17
21
20
12
12
18
best
resulting
odginal transects
indication of the success
function analysis; Variable
a
HI
I
HI
3
0
13
0
IV
1
1
0
0
8
0
>50
MA
1
n
Discriminating
Variables
2
correct
0
0
0
2
Zone
83
outer reef flat
inner reef flat
crest +
crest + slope
81
100
50
>50
F
= 8.2.
= 15,
o
P<
A"-U,"".H...,1
et
t:l<'I'"'I-.<:>I1"
(1986) with
(1988) with re>nor81
One of the first problems encountered in this study was the time consuming nature
the
sampling procedure which resulted
only 10 ground control points being surveyed
benthic cover. This is a relatively sman number of points even though the total transect
was 200m, which covers 3 to 4
for Landsat l\ISS
to 10 pixels for
has to
out
""".....-".-r.
V ......LIl ....' ....
Another
"IV,",'IJ,",'
to
crest and slope,
areas are ecologically
MSS.
'.'''-Hl.",nT
did separate biotic
abiotic
geomorphological zones also.
and so .. ""'''''''''-,,'VA ......... .....,.
the raw abundance
of
2 has not been the subject of . . . ""' ..........'..... . ........ j''''''Jl •. ...., .....
1
for
imagery data
is not
terrestrial areas
SPOT mapping, Forster et al., 1987). The lineaments evident in the outer reef flat zone
are not detected in
LandsatMSS data because of the difference
spatial resolution.
are sand channels between the strips of hard substrata which may
( 1982)
.Il
('",,,,.. ,,. ... 01,,.
1
press.
Dixon, W.J. and Brown, M.B. (Editors) 1979. BMDP-79 Bionledical computer
P-series. University of California Press, Los Angeles, 880pp.
Forster, B.C., Ttinder, J.C Can"olL
,FaITington.
. Kwoh, L., Nicholson,
Holden. G.F.H. 1987. Mapping research using SPOT digital data.
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490-499.
q
Hill, G.J.E. and Kel1y. G.D. 1987. Landsat TM versus MSS for land cover
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Septenlber 1987. Volume I: I
Hopley,
1982.
York,453pp.
New
geomorphology of the
Jupp, D.
,Heggen, S ,Mayo,
B.A. 1985a. The BRIAN handbook ...............
Natural Resource Series 3, 51 pp.
A ..........
Jupp, D.L.B. and Mayo. K.K. 1982. The use of residual images
analysis. Photogram. Eng.
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• Kenchington, R.A. and
Jupp, D.L.B., Mayo, K.K., Kuchler, D.A., Claasen. D.van
Guerin,
1985b. Remote sensing for planning and managing the Great
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and Menor,
Kuchler, D.A, Maguire, C" McKenna, A., PtiesL
ITC
1
survey method for verification of LandsatMSS image
217-223.
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press.
Reichelt, R.E .. Loya, Y. and Bradbury, R.B.
benthic communities on two coral reefs
73-79.
Ipp.
Richards. J.A. 1986. Remote sensing digital analysis.
Smith, V.E. 1975. Thematic
'Y"Q1'1Ih-r~.,.n.'" Institute
Science
1Oth lnt. Symposium on Remote Sensing of
10pp.
Veron, J.E.N. 1986. Corals of Australia and
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Indo-Pacific. Angus