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A glimpse of Digital Earth: maps of the elusive intertidal zone

Once fully operational, the new Digital Earth Australia service will be a world first national data service for satellite imagery. Based on new research conducted by Geoscience Australia, we have just witnessed a glimpse of the powerful services Digital Earth Australia will soon be providing.

Using 28 years of Landsat data, a research team from the National Earth and Marine Observation Branch of Geoscience Australia has created a continent-wide intertidal zone extent map for the whole of the Australian coast—some 50,000 kilometers. The results are not only stunning to view, but contain valuable insights into coastal habitats.

The intertidal zone is that elusive strip of land that the low tide reveals and the high tide conceals. This zone is a rich ecosystem that provides habitat for migrating shorebirds and is a first line of defense against extreme storm events. However, pressure is mounting on this zone from sea-level rise and anthropogenic sources such as land reclamation and aquaculture.

The intertidal zone of Magnetic Island in Queensland. Photo credit: Leo Lymburner

Due to its transient nature, the intertidal zone has been particularly troublesome to map and study. Thankfully, Geoscience Australia’s new automated approach has overcome that.

Dr Stephen Sagar from Geoscience Australia led the team that created the Intertidal Extents Model, also known as ITEM. The team’s mapping breakthrough was made possible by the United States Landsat satellite data spanning from 1987–2015 and a new automated workflow. The dataset is available online as open data through Geoscience Australia’s data portal.

In the case of ITEM, ensemble processing enabled tidal flat extent maps to be made for the first time for Roebuck Bay and the Kimberley Coast, two ecologically important regions in Western Australia where cloud cover had thwarted earlier satellite mapping attempts.

This held for the north as well. As Sagar described: “Having a long and complete data record was vital to enable ITEM’s continental scale approach to be valid in Northern tropical regions where there is often extensive cloud cover.”

ITEM is the first continent-wide intertidal zone extent map for the whole of the Australian coast, and covers some 50,000 kilometers.

Hint of what’s to come

ITEM was one of the datasets promised when Digital Earth Australia was first announced in May 2017 as part of Australia’s Federal Budget. Less than a month later, Geoscience Australia has already delivered on this part of the promise.

Digital Earth Australia (DEA) aims to evolve upon the award winning Australian Geoscience Data Cube backed by $15.3 million of new federal funding. Within just two years’ time, Digital Earth Australia has the ambitious plan to provide open access to 10 metre resolution imagery nationwide, updated every five days.

In addition to ITEM, Digital Earth Australia, also plans to provide open access to water observations, fractional cover (FC), a normalised difference vegetation index (NDVI) and surface reflectance. Together these datasets will better inform the likes of agriculture, environmental monitoring, coastal erosion studies and land research.

The ITEM relative extents model for Cape Palmerston in Queensland. Image credit: Sagar et al, 2017

A key example of imagery en-masse

The novelty of Geoscience Australia’s work on the intertidal zone is not only that the nationwide intertidal zone map product is a first, but the methodology used is an entirely new approach. The team used a “change of time” procedure more commonly found in modern financial models.

By adding a tidal stage attribute for each of the 221 coastal data tiles in the dataset using tidal prediction software, and knowing the date and acquisition time of each Landsat data pixel, the data could be organised based on tidal stage.

“The Landsat data record has been key to the ITEM methodology,” Sagar said. “Having such an extensive and dense time series of data has enabled us to partition the data into discrete tidal stages, and still be able to deal with issues such as cloud and cloud shadow—that is particularly crucial when dealing with the different conditions we encounter across the continent.”

Talbot Bay in the Kimberley—with the ITEM map overlay identifying the area’s intertidal zone and onshore reefs. The ITEM map for this region is also part of the Northern Australia bathymetry grid project. Image credit: Stephen Sagar

With each Landsat pixel’s tidal stage established, a Normalized Difference Water Index (NDWI) was then calculated to deduce whether or not land in the intertidal zone was covered with water at that tidal stage.

With the all observations sorted by tidal stage, the team could then take each pixel’s median-based NDWI value for every tidal stage category. Combining the resulting nine maps yielded the first continent-wide tidal extent map for Australia.

Since tidal extent is related to the tidal range and the shoreline elevation, the data set also provides relative topography, allowing the researchers to derive coastal elevation models which may prove useful for future hydrodynamic modelling and current forecasting.

 

Introducing Digital Earth Australia

Benefits already coming to fruition

The researchers expect that their intertidal extent maps will contribute to coastal environmental monitoring, habitat mapping and geomorphological studies.

“ITEM is already being used by the Queensland government to assist in their inter-tidal and sub-tidal habitat mapping program for central Queensland,” Sagar relayed.

And through collaboration with James Cook University, ITEM has contributed to the development of the Northern Australian 100-meter bathymetry grid, a water depth chart sourced from all available bathymetry measurements for the region.

“ITEM has been able to provide topographic and elevation information in this remote region of Australia, characterized by large tidal ranges and extensive intertidal regions,” Sagar explained. “Many of these regions were previously unmapped.”

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