RETIRED Water Observations from Space Filtered Statistics 25m 2.1.5  

Authors / CoAuthors

Abstract

<b>This record was retired 15/09/2022 with approval from S.Oliver as it has been superseded by eCat 146091 DEA Water Observations Statistics (Landsat)</b> In previous versions of WOfS, the basic water classifications, statistical summaries and confidence products were contained within one product with several datasets. As of version 2.1.5, WOfS is split into three products: Water Observation Feature Layers (WO_25_2.1.5), Summary Statistics (WO-STATS_25_2.1.5), and Filtered Summary Statistics (WO-FILT-STATS_25_2.1.5). This product is Water Observations from Space - Filtered Statistics (WO-FILT-STATS), consisting of a Confidence layer that compares the WO-STATS water summary to other national water datasets, and the Filtered Water Summary which uses the Confidence to mask areas of the WO-STATS water summary where Confidence is low. The Filtered Water Summary provides the long term understanding of the recurrence of water in the landscape, with much of the noise due to misclassification filtered out. WO-FILT-STATS consists of the following datasets: Confidence: the degree of agreement between water shown in the Water Summary and other national datasets. The Confidence layer provides understanding of whether the water shown in the Water Summary agrees with where water should exist in the landscape, such as due to sloping land or whether water has been detected in a location by other means. Filtered Water Summary: A simplified version of the Water Summary, showing the frequency of water observations where the Confidence is above a cutoff level. This layer gives a noise-reduced view of surface water across Australia. Even though confidence filtering is applied to the Filtered Water Summary, some cloud and shadow, and sensor noise does persist.

Product Type

dataset

eCat Id

121061

Contact for the resource

Keywords

( Theme )

• thematic data

( Theme )

• water

( Theme )

• remote sensing

( Theme )

• Landsat 5

( Theme )

• Landsat 7

( Theme )

• Landsat 8

( Theme )

• flood

• Published_External

Publication Date

2019-01-18T03:29:48

Creation Date

2018-05-29T00:00:00

Security Constraints

Australian Government Security ClassificationSystem    

Classification - unclassified

Legal Constraints

Creative Commons Attribution 4.0 International Licence    

Access - license

Use - license

Status

retired

Purpose

The primary purpose of the WOfS product suite is to help understand where flooding may have occurred in the past. This has application in emergency management and risk assessment. The product has many secondary uses. For example the products provide an indication of the permanence of surface water in the Australian landscape by showing where water is observed rarely in comparison to where it is often observed. This has application in water management and mapping. The products have also been used for wetland analyses, water connectivity and surface-ground water relationships.

Maintenance Information

notPlanned

Topic Category

geoscientificInformation

Series Information

Lineage

The WOfS product is a key component of the National Flood Risk Information Portal (NFRIP), developed by GA. The objective of Water Observations from Space is to analyse GA's historic archive of satellite imagery to derive water observations, to help understand where flooding may have occurred in the past. The collection of many hundred thousand WOFLs that make up WOfS are too cumbersome to display easily. WO_STATS provides the mechanism to present and deliver WOfS in a more easily digestible form, and provides understanding of water in the landscape. WO-FILT-STATS provides extra information to give a confidence in whether water is likely in the locations shown by the other WOfS products and provides a final, "cleaned" summary of water over time. WO-FILT-STATS is created from the WOfS-STATS statistics (WO-STATS_25_2.1.5) derived from water classification (WO_25_2.1.5). Reviews of prototype products identified the need to indicate the level of confidence for the surface water observations. The confidence level will help the user to distinguish between unusual but valid water detections (such as flood plains which might only be observed as water once in the 15 year interval) and ‘false positives’ which can be caused, for instance, by steep shady slopes. The confidence level was determined through a multiple logistic regression of water observations against several factors that would either support or contradict the finding of water being present at the site. The factors comprise: MrVBF, a multi-resolution valley bottom flatness product (Gallant et al., 2012) derived from Shuttle RADAR Topographic Mission (SRTM) data as part of the Terrestrial Ecosystems Research Network (TERN). Surface water pixels identified in valley bottoms were more likely to be positively detected. Slope derived from SRTM Digital Surface Models. Water pixels on a slope were considered less plausible than those on a flat surface. MODIS Open Water Likelihood (OWL) (Ticehurst et al, 2010) provides a plausibility based an independent water detection algorithm employing the MODIS sensor. If both detection algorithms agree on the presence of a surface water pixel, there is a greater plausibility that the detection is correct. Observation Frequency (P), the number of observations of water as a fraction of the number of clear observations of the target pixel. P is high for more permanent water bodies. Built-Up areas indicating areas of dense urban development. In such areas the water detection algorithm struggles to cope with the deep shadows cast by multi-story buildings and the generally noisy spectral response created by structures. The Built-Up layer is derived from the Ausralian Bureau of Statistics ASGS 2011 dataset, for urban centres of populations of 100 000 and over. Once calculated the Confidence is used to filter the Water Summary from WO-STATS where confidence is <10% to create the Filtered Water Summary. Record retired 15/9/22 per approval TRIMM D2022-3489

Parent Information

Extents

[-44, -9, 112, 154]

Reference System

Spatial Resolution

25m

Service Information

Associations

Downloads and Links

Geoscience Australia Landsat Water Observation Statistics Collection 3  

Source Information