About Water Observations from Space
Water Observations from Space (WOfS) is a web service displaying historical surface water observations derived from satellite imagery for all of Australia for the period of 1998 to 2012. By June 2015 this service will be extended to enable users to view data on surface water observed by satellite across Australia since 1987. WOfS displays the detected surface water from the Australia-wide Landsat-5 and Landsat-7 satellite imagery archive. The aim of WOfS is to better understand where water is usually present; where it is seldom observed; and where inundation of the surface has been occasionally observed by satellite.
In the first phase of this project, delivered in November 2012, the service showed a proof of concept of how satellites can be used to detect surface water across Australia for specific periods between 2000 and 2012. Following further development, the new version of this service - which is still being reviewed and tested - now shows surface water observed by the Landsat-5 and Landsat-7 satellites for all of Australia from 1998 to 2012.
Water is detected from the satellite images using an automated flood mapping algorithm created by Geoscience Australia. The water detected for each location is summed through time and then compared to the number of clear observations of that location (i.e. observations not affected by cloud, shadow or other quality issues). The result is a percentage value of the number of times water was observed at the location. This provides relatively seamless historical water coverage for Australia, as observed by satellite. The colouring of the summary layer indicates how often water was observed in each cell of a 25 metre by 25 metre grid across Australia. Possible floods appear in the low values (red to yellow colours) while consistent water bodies such as lakes and dams have high values (blue to purple colours).
This latest version of WOfS displays, for each grid cell:
- the number of clear satellite observations over the period (1998-2012);
- the number of occasions water was detected;
- the percentage of clear observations on which water was detected;
- the confidence (or probability) that a water observation in this location is correct. This is a percentage, based on a number of factors including the slope of the land and the existence of other corroborative evidence.
More information on Geoscience Australia's Water Observations from Space product is available in the WOfS FAQs.
Limitations of the Information
The automated surface water detection algorithm, which has been developed by Geoscience Australia, can mistakenly label as "water" large buildings; cloud shadow; large uniform black tarpaulins; or snow. The algorithm is designed to locate large areas of water and as a result may miss small water bodies. To understand how valid the water observations are, every point in the WOfS product has an associated confidence value to help understand how likely it is that water was observed in a location. In a small number of cases - for example where large buildings or black material such as tarpaulins are present - the calculated confidence values can be over-estimated.
Observation of the Earth by the satellites used for this service depends on clear skies. Furthermore, the satellites do not observe all places every day. The Landsat satellites, which are the basis for this service, view a given 185 kilometre wide strip of Australia only every 16 days. As a result, the observations show only what was visible on the day of the satellite pass. As a result, not all historical floods will have been observed by satellite.
The satellite archive used for this service is of limited duration (currently 1998-2012), and subject to the cloud and repeat coverage restrictions noted above. In addition, Australia is subject to wide variations in weather and climate. Therefore the absence of prior water observations in a particular location does not provide certainty that surface water will never be observed in future.
The probability that surface water may appear at a given location may vary over time due to changes in drainage and other infrastructure (such as dams). Where such changes have occurred, the historical water observations for that location may no longer give a true picture of the future probability of surface water being observed.
- The “clear observations” count is not correct for steep slopes. In hilly areas the clear observation counts reads as zero. This will be addressed in future versions.