Citation
Hossain, Mohammad Shawkat
(2015)
Development of mapping methods for seagrass meadows in Malaysia by using landsat images.
Doctoral thesis, Universiti Putra Malaysia.
Abstract
In Malaysia, seagrasses commonly inhabit shallow intertidal waters, lagoons,
mangrove, coral reef and shoal in subtidal zones. The seagrass meadows that
previously inhabited the coasts of Peninsular Malaysia (West Malaysia) and
Sabah (East Malaysia) were extensive and now have become sparse or
scattered due to natural and anthropogenic disturbances. Scientists and
managers require a fast and low-cost approach to map and assess the habitat
loss or potential damage to the seagrass resources. A critical review of 195
studies revealed that, in the past four decades, advances in the application of
remote sensing (RS) methods, notably using Landsat imagery, were identified
for seagrass habitat mapping. Mapping capabilities of Landsat were not tested
in different tidal regimes, characterizing seagrass habitats in relation to water
turbidity and depth regimes, and understand spatiotemporal dynamics from
multi-date images due to lack of appropriate methods and data, including
unresolved Scan Line Corrector (SLC)-off data gaps in Landsat 7 Enhanced
Thematic Mapper (ETM)+ images.
The specific objectives of this study were to: (1) characterize the research on
seagrass RS methods through a review of the peer-reviewed literature, (2)
assess the performance of Landsat 7 ETM+ SLC-off gap-filling methods and
image enhancement techniques (ETs) with different water depths for
methodological improvement in seagrass resource mapping and monitoring,
and (3) apply mapping approach on seagrass habitat, water turbidity and
relative water depth mapping and monitoring.
A statistical assessment and an evaluation of the twelve SLC-off and four SLCon
images covering seagrass meadows of Sungai Pulai estuary, Johor,
Malaysia were conducted for data loss estimates. This analysis revealed a 2%
systematic error attributable to a gradual increase of SLC-off stripes from the
central nadir path towards the edge of the scene. The random shifting of SLCoff
stripes caused a 11.07 ha Tanjung AdangLaut shoal (TALS) completely
invisible. The next study was focused on assessment of the geometric and
radiometric fidelity of images reconstructed by three potential gap-filling
methods: (1) Geostatistical Neighbourhood Similar Pixel Interpolator (GNSPI), (2) Weighted linear regression algorithm integrated with Laplacian prior
regularization, and (3) Local Linear Histogram Matching methods. The
statistical measures of reconstructed images were in favor of the use of GNSPI
as opposed to other gap-filling techniques for Sungai Pulai estuary seagrass
distribution mapping.
To assess the variation in performance of Landsat image enhancement for
Sungai Pulai estuary seagrass maps, with different Mean sea-level tide heights
(MSLTHs), a comparison was conducted between histogram equalization (HE)
and manual enhancement (ME) based mapping approaches. The
enhancement techniques were applied on true-color composites (red, green,
and blue layer stacks) of thirty-three Landsat images (1989-2014) with
MSLTHs between -0.281 and 0.234 m. The assessment found that ME
substantially improved image quality compared to the HE with MSLTH
thresholds between -0.218 m and -0.085 m. ME improved visual interpretation
of Landsat images for seagrass detection and distribution mapping of
Merambong shoal (MS), Tanjung Adang Darat shoal (TADS), TALS, and
Seluyong mudflat (SMF).
An integrated mapping approach, combining Landsat ME and seed pixel
regional growing tool was examined to delineate seagrass boundary
accurately. This approach was found suitable (with >75% overall accuracy) to
map five classes-of-interest, i.e., seagrass, land, sand/mud, human settlement,
coral and coral rubble for twelve islands of Coral Triangle Initiative (CTI),
Sabah. The resulted map estimated seagrass areal coverage to be 274 ha, of
which most seagrass meadows occurred in relatively shallow water areas
covering about 158 ha. Issues of spatiotemporal changes in seagrass habitat
were addressed through assessing the ability of the integrated mapping
approach on multi-date Landsat images for mapping and monitoring seagrass
resources of Punang-Sari estuary, Lawas, Sarawak, Pengkalan Nangka
lagoon, Kelantan, and Paka lagoon, Terengganu of Malaysia. Applying this
integrated approach on forty-nine Landsat 5, 7 and 8 images, produced an
accurate multi-date seagrass habitat maps. Additionally, the results indicated
that a noticeable loss of seagrass habitats at varying magnitude occurred
between 2000 and 2014 for Punang-Sari, between 1998 and 2014 for
Pengkalan Nangka, and between 1988 and 2014 for Paka. The natural event
mainly sand shifting was the main cause of seagrass loss for Punang-Sari
Lawas. Coastline change was identified as the most significant factor that
caused seagrass spatial cover loss of the Pengkalan Nangka lagoon.
The mapping approach and the map products produced in this study will
provide a useful tool for detection, distribution mapping and monitoring
changes of seagrass and associated resources for coastal management and
conservation programs.
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