This is a proof of concept web service displaying trial study areas of historical flood mapping derived from satellite imagery. Over the next two years this service will be developed into a nationwide portal to ultimately provide data on flooding which has been observed by satellite across Australia since 1987.

In this initial phase, the service shows a summary of water observed by the Landsat satellite and the satellite mounted sensor, MODIS, across Australia for specific periods between 2000 and 2012. The MODIS sensor images all of Australia almost every day, but much of the imagery captured contains large amounts of cloud. To obtain the best, cloud-free result, eight days of images are combined into a single image. The national, low resolution layer in this service is produced from all eight-day composite MODIS images from 2000 to 2012. The Landsat-5 satellite only images the same 185 kilometre wide strip of Australia every 16 days, and many of these days are affected by cloud cover. As a result, Landsat cannot produce a continuous series of images through time in the same way MODIS does. The three study area layers in this service are produced from Landsat-5.

The water was detected from the satellite images using an automated flood mapping algorithm created by Geoscience Australia. One national coverage and three study areas are shown. Click on the magnifying glass next to each layer to zoom to the corresponding area.

The National, Low Resolution layer displays a summary of how often water was observed across Australia by the MODIS sensor from 2000 to 2012. The colouring of the summary layer shows how often water was observed in a 500x500 metre grid across Australia from 573 MODIS images ranging from 2000 to 2012 (roughly one image every eight days). The higher the number shown in the layer, the more often water was observed between 2000 and 2012. This means that occasional events, including floods, appear in the low values (red to yellow colours), while consistent water bodies, such as lakes and dams, will have high values (blue to purple colours).

Three Study Areas are shown and can be expanded by clicking on their title in the layer list. These study areas use the Landsat-5 satellite to detect water in a 25x25 metre grid across each study area. Each study area layer set includes a water summary displaying how often water was observed across the area by the Landsat-5 satellite for a set of years. Similar to the national, low resolution summary, 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).

Also, each study area set includes individual flood extents for specific dates when large-scale flooding was detected. Each layer can be switched on and off by ticking or un-ticking the corresponding box and the corresponding layer slider allows the layer to be made transparent. This helps compare one flood with another and with the full water summary for each study area.

The study areas and observation details are:

• Study Area 1, Condamine River system between Condamine and Chinchilla, Queensland, observed between 2006 and 2011 from 49 images.
• Study Area 2, Northwest Victorian rivers between Shepparton and Kerang, observed between 2006 and 2011 from 36 images.
• Study Area 3, North Queensland rivers, near Normanton, observed between 2003 and 2011 from 112 images.

Limitations of the Information

The automated flood mapping algorithm, which has been developed by Geoscience Australia, can confuse cloud shadows and snow with flood water, so some areas shown as water may be incorrect. The algorithm is designed to locate large areas of water and as a result may miss small water bodies. This is a trial dataset and has not been validated.

Observation of the Earth by satellite depends on clear skies. Where an area is too cloudy, observations may be missed. As a result, not all floods will be observed.

Satellites have specific, scheduled observation times. MODIS observes all of Australia approximately twice daily, while Landsat-5 views the same 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, which can limited by cloud cover.