Satellite-derived bathymetry for nearshore benthic habitats in Timor-Leste

Bathymetric data derived from multispectral, high-resolution (2 m) satellite imagery from DigitalGlobe’s WorldView-2 satellite to provide near complete coverage of nearshore terrain along the northern coastline of Timor-Leste, including the district of Oecusse to the west of mainland Timor-Leste and Atauro Island. Satellite images acquired by the NOAA Coral Reef Ecosystem Program (CREP) for the region are from Feb 9, 2010 to October 2, 2013. Methods used by CREP to estimate depths from the WorldView imagery were adapted from instructions developed by Kyle Hogrefe for using IKONOS imagery to derive seafloor elevations in optically clear water (Ehses and Rooney 2015).

Bathymetry values shallower than ~20 m were derived by gauging the relative attenuation of coastal, blue, green and yellow spectral radiance as a function of depth. A multiple linear regression analysis of coastal, blue, green and yellow band spectral values against in-situ depth determined the variables of y-intercept, coastal, blue, green and yellow slope values. The variables for each band were then used in a multivariate slope intercept equation to derive depth over the imagery. Variables and combinations of the bands were adjusted to improve the statistical accuracy and spatial coverage of the final derived bathymetry product. Digital image processing to derive depths was conducted with ENVI, and editing and integration was performed using Esri's ArcGIS.

The method assumes uniform water clarity but deviations from that condition made extraction difficult in water depths greater than 20 m. Results show that biotic material, or sediment in the water column skewed results shallower if the material has a high albedo and deeper if the material has a low albedo.

In 2011, NOAA and the United States Agency for International Development (USAID) Timor-Leste Mission formed a 5-year bi-lateral partnership agreement from 2012 to 2016, in support of the Government of Timor-Leste—particularly the Ministry of Agriculture and Fisheries (MAF)—by addressing significant information gaps in their nearshore marine ecosystems. Specifically, MAF asked NOAA to address the following questions:

1. Where are the nearshore marine resources?

2. What are the nearshore marine resources?

3. How are they changing over time and what threats are potentially causing these changes?

4. What approaches are needed to manage and conserve the nearshore marine resources?

With financial support from USAID and in-kind support from the NOAA Coral Reef Conservation Program (CRCP), the NOAA Coral Reef Ecosystem Program (NOAA-CREP) conducted baseline ecosystem assessment surveys to inform ecosystem-based management planning in Timor-Leste. The primary activities conducted by NOAA to answer these questions included:

1. satellite mapping of nearshore habitats,

2. conducting coral reef ecosystem assessments,

3. establishing ecological baselines for climate change, and

4. building management capacity by developing a spatial data framework

The bathymetry data described herein resulted from the first activity to conduct satellite mapping for the nearshore waters around Timor-Leste.

Pacific Islands Fisheries Science Center. 2017. Interdisciplinary baseline ecosystem assessment surveys to inform ecosystem-based management planning in Timor-Leste: Final Report. Pacific Islands Fisheries Science Center, PIFSC Special Publication, SP-17-002, 234 p. https://doi.org/10.7289/V5/SP-PIFSC-17-002.

The method to derive depths from WorldView-2 satellite imagery is documented in the following NOAA publication:

Ehses, J. S., and J. J. Rooney. 2015. Depth derivation using multispectral WorldView-2 satellite imagery. U.S. Dep. Commer., NOAA Tech. Memo., NOAA-TM-NMFS-PIFSC-46, 24 p. doi:10.7289/V5668B40

Bathymetry raster files, in geotiff format, derived from WorldView-2 satellite imagery from coastal waters of Timor-Leste. Pixel values reflect the estimated depth in meters.

While every effort has been made to ensure that these data are accurate and reliable within the limits of the current state of the art, NOAA cannot assume liability for any damages caused by errors or omissions in the data, nor as a result of the failure of the data to function on a particular system. NOAA makes no warranty, expressed or implied, nor does the fact of distribution constitute such a warranty.

Timor-Leste

Not applicable

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NOAA Coral Reef Ecosystem Program (CREP) Data Sharing Recommendations, version 9.0 updated August 12, 2015:

CREP welcomes the opportunity to collaborate on research issues contributing to the scientific basis for better management of marine ecosystems. CREP has a very diverse set of field activities that generates large volumes of data using an array of data collection protocols.

The following recommendations are for your consideration as you use this data:

1) Data analyses should take all field exigencies into account. The most effective way to do this would be active collaboration with CREP principal investigators.

2) In all presentations, product releases, or publications using data generated by CREP, proper acknowledgement of both CREP and the individuals responsible for data collection is expected. Citing the DOI (if available) is preferred, a non-DOI example is listed below.

3) If you collect or generate data for the same study areas, CREP requests that you share relevant information on complimentary data collections.

4) Those receiving data are strongly urged to inform the CREP Data Management Team of any errors and discrepancies that are discovered during the course of using these data. They are further urged to bring to the attention of the Team all problems and difficulties encountered in using these data. This information is necessary in order to improve the collections and to facilitate more efficient and economical data processing and retrieval. The users are asked to supply copies of any missing data that may be located, and to provide information as to significant subsets and special aggregations of data that are developed in using the material provided.

Example citation:

"This publication makes use of data products provided by the Coral Reef Ecosystem Program (CREP), Pacific Islands Fisheries Science Center (PIFSC), National Marine Fisheries Service (NMFS), National Oceanic and Atmospheric Administration (NOAA), with funding support from the United States Agency for International Development (USAID). The analysis and interpretations presented here are solely that of the current authors.”

Data can be accessed online via the NOAA National Centers for Environmental Information (NCEI) Ocean Archive

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Please cite NOAA Coral Reef Ecosystem Program (CREP) when using the data.

Example

Coral Reef Ecosystem Program; Pacific Islands Fisheries Science Center (2017). Satellite-derived bathymetry for nearshore benthic habitats in Timor-Leste. NOAA's National Centers for Environmental Information. Dataset. https://www.fisheries.noaa.gov/inport/item/46150

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ENVI was used to process the satellite images, and maps and data were developed using ArcGIS. Spatial reference was UTM Zone 51S.

Timor-Leste project on PIFSC network: \\picGoldFish\General\ProjectGroup\Coral Triangle\Timor Leste\Spatial_Data_Framework\

The program Microsoft Excel was used for the multiple linear regression analysis to provide original variables for depth derivation. The multivariate slope intercept formula used to derive depth was Depth=Yint+(Mcoastal)(Xcoastal)+(Mblue)(Xblue)+(Mgreen)(Xgreen)+(Myellow)(XYellow).

Multiple linear regression analysis using subsampled athymetry validation points.

Nearshore waters along the entire coastline of Timor-Leste have complete coverage of WorldView-2 satellite imagery. There are, however, holes in derived depths resulting from areas of high water column turbidity, clouds, whitewater, etc. that were masked out of the analyses due to a lack of reliable imagery of the seafloor for those areas. No bathymetry was generated for the southern coastline of Timor-leste because no depth data were available to calibrate the imagery and to subsequently validate the depths.

Pre-processing of all WorldView-2 satellite images was performed using the same methods, to produce a rectified, deglinted image, with radiance and depth invariant data layers.

Based on the error analyses, derived bathymetry was clipped where statistical results were poor.

NOAA IRC and NOAA Fisheries ITS resources and assets.

Bathymetry data were derived for nearshore waters along the northern coastline of Timor-Leste, and the coastlines of Oecusse district and Atauro Island, from WorldView-2 satellite imagery following methods developed by Ehses and Rooney (2015).