Mapping Coral Reefs

Mark Altaweel


We have become a lot better at mapping our world with different airborne and satellite technologies. There are, however, areas where we still face a great challenge.

One of these is mapping coral reefs in high resolution. Recently, scientists have been developing better ways to fuse data from different satellite systems and data capture, while also developing classification algorithms that make corals more evident using different sensors.

Coral reefs are known to be very import for biodiversity in ocean systems, in a way very similar to rainforests, where they also provide food to aquatic ecosystems and humans living near them worldwide.

Over the last year, scientists have been increasingly alarmed by the pace of change and threats to coral reefs on a world-wide scale.

Picture of corals taken underwater.
Healthy Elkhorn coral (Acropora palmata) near unpopulated Buck Island, U.S. Virgin Islands. Elkhorn coral is one of many important reef-building species that create 3D structure on the seafloor. Coral reef structure provides habitat for marine life and helps break up waves as they approach the coastline. Photo: Curt Storlazzi, USGS. Public domain.

Similar to rainforests, pollution, over-exploitation, and climate change are threatening coral reefs. However, we have only mapped a small percentage of coral reefs and we have relatively limited resources monitoring their health worldwide.

In October of 2018, a new atlas, called Allen Coral Atlas, has provided the first high resolution map of coral reefs around the world at a resolution of about 3.66 meters or 12 feet.[1]

The Allen Corel Atlas provides access to high resolution mapping of coral reefs around the world.
The Allen Corel Atlas provides access to high resolution mapping of coral reefs around the world.

The project is not complete and there are some remaining challenges.

First, classifying signatures for coral reefs is not a straight task, as different coral reefs may have differing composition and their presence in different water signatures also affects how they can be read. Research has often focused on limited areas where corals exist, thus mapping at a global scale requires knowledge of regions where potentially coral reefs exist but have not been observed yet.[2]

Capturing the range of diversity requires knowledge as to what reflectance signatures coral reefs may be best classified in order to utilize more typical classification schemes from multiple satellite sources. Using satellites from planet labs allows different systems and observation data to train automated classification algorithms to train classification. The constant classification and updates help to create a real-time map of the world’s coral reefs.

There have been also challenges in relation to the staff, as the main advocate and driving force for the project, globally renown scientist Ruth Gates, had recently passed away. Paul Allen, the well known Microsoft co-founder, who funded the effort, had also passed away in 2018.

Nevertheless, despite these funding and leadership challenges, mosaics and updates are continuing and at this rate by 2020 the first complete and updatable high resolution map of coral reefs should be completed.[3]

While the new Allen Coral Atlas effort is ambitious and is likely to be the most detailed effort to date, there were earlier efforts that also tried to map known coral reefs around the world.

In 2001, there was an initial atlas released, which also came as a volume, that discussed and showed known coral reef areas. However, this is still a small fraction relative to the likely known coral reefs around the world. Many of these corals are those that are evident with the naked eye, whereas many corals may lay hidden in areas we may not expect them to exist.[4]

New types of algorithms in recent years also utilise object-based techniques, rather than depending on pixel resolution, and support vector machine (SVM), including other classification techniques, that have proven useful for coral reef identification.[5]

One great challenge, to map the world’s coral reefs in high resolution, is about to resolved.

This is a mixed blessing, as, on the one hand, it will provide scientists with a way to monitor more closely the health and well-being of corals around the world.

On the other hand, we are now likely to see threats that show the great danger coral reefs are in. Coral reefs are critical to the economy of communities, health for ecosystems, and natural resources they provide. They are also very sensitive to change in water quality and temperature.

The satellite data we now will have allows scientists to monitor their health at a high accuracy for the first time.


[1]    For more on the Allen Coral Atlas project, see:

[2]    For more on classification techniques for coral reefs, see:  Iovan, C., Ampou, E., Andrefouet, S., Ouillon, S., & Gaspar, P. (2015). Change detection of coral reef habitats from multi-temporal and multi-source satellite imagery in Bunaken, Indonesia. In 2015 8th International Workshop on the Analysis of Multitemporal Remote Sensing Images (Multi-Temp)(pp. 1–4). Annecy, France: IEEE.

[3]    For more on the methods deployed, see:

[4]    For more on the earlier effort for mapping coral reefs, see: Spalding, M., Ravilious, C., & Green, E. P. (2001). World atlas of coral reefs. Berkeley: University of California Press.

[5]    For more on some classification techniques that have proven useful for coral reef identification, see:  Wahidin, N., Siregar, V. P., Nababan, B., Jaya, I., & Wouthuyzen, S. (2015). Object-based Image Analysis for Coral Reef Benthic Habitat Mapping with Several Classification Algorithms. Procedia Environmental Sciences, 24, 222–227.

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.

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