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NASA Goddard's Hydrology Laboratory is working to operationalize near real-time global flood mapping using available satellite data resources -- currently the twice daily overpass of the MODIS instrument, on the Terra and Aqua satellites. We are working to include additional data sources, such as Landsat and radar, to improve coverage and accuracy.
This work builds on the long-time expertise and efforts of the Dartmouth Flood Observatory (DFO) to map floodwater extent in detail for active floods. The DFO website provides additional detail, additional products, and archives of historical flood maps. This NASA website provides links and information about the automated daily flood and surface water products produced at NASA Goddard.
Note:
The MODIS Flood 3 Day Image Service is the representing the MODIS Water Product (combines both MFW and MSW, raster only). There is no indication provided of where there is insufficient clear data in the given product to determine water extent. Thus, these products only indicate where water has been detected, but the absence of a water polygon cannot be interpreted to mean there was no water present in a given area; it may simply have been sufficiently cloudy over the composite period for the required number of water observations to be met (the Y in composite indicator XDYO). The MWP product (below) attempts to address this deficiency, and may eventually replace MFW and MSW. These vector products can also be extremely resource intensive to generate from the source raster; a partly cloudy scene over water, or a tile over mountains with terrain shadow issues, can have tens of thousands of polygons, if we were to convert to shapefile or KMZ. Thus, we typically skip vector product generation if the number of polygons is greater than 20000. Currently delivered only in geotiff raster format.
We detect water using an algorithm developed by Bob Brakenridge of the Dartmouth Flood Observatory. This algorithm uses a ratio of MODIS Band 1 and Band 2, and a threshold on Band 7 to provisionally identify pixels as water. We then composite the water detections over the product window (2 days, typically). If a pixel is identified as water over several (2 or more) observations during the product window, it is then definitively marked as water, and output in the MSW, or "MODIS Surface Water" product. 2 (or more) observations are required because cloud shadow can appear quite spectrally similar to water.
In cases where cloud shadow occurs in the same spot in 2 observations, the product may incorrectly flag such areas as water. A 3 observation requirement helps further, but also increases latency of the product. At the moment, we are using the 2 observation requirement as a balance between accuracy and timeliness. Finally, the detected water is compared to a reference water layer that shows "normal" water extent, and any pixels found outside the normal water extent are marked as flood, and output in the MFW ("MODIS Flood Water") products.
The Image Service is published using a Raster Mosaic populated with “MWP” GeoTiffs collected from the NRT Global Flood Mapping Data Viewer site.
Data Visualization:
MODIS Flood layer is represented using Unique Values Ranging from 0-3.
0 : Insufficient data to make water determination (cloudy, missing images, swath gaps, or bad data values).
1 : No water detected.
2 : Water detected AND coinciding with reference water (e.g., not flood).
3 : Water detected, beyond reference water, so may be flood.
Further Reference:
Learn more about MODIS Flood Products
Purpose/Summary:
Flood Detection Map, MODIS Flood 1 Day (1D1OS), Updated Daily
The MODIS NRT Global Flood product (MCDWD) reports flood and surface water detected by NASA's MODIS optical instrument. This instrument is carried on two NASA satellites, Terra and Aqua, and thus provides twice-daily near-global observations. It is an optical instrument so cannot see through clouds.
The product is delivered as 1, 2, and 3-day composites. These alternatives are provided to minimize the influence of clouds on the product: clouds will block the satellite view of the surface (potentially causing false-negatives where the product does not detect water), and cloud shadows can be erroneously detected as water (potentially causing false-positives where the product predicts water or flood occurrence when there is none).
The 1-day product only uses observations from the product date, and will report water if it is detected by any single observation (in general over one day, there will be two observations, one from each satellite).
The 2- and 3-day products combine observations over the current plus previous day (2-day), or current plus two previous days (3-day), and require multiple water detections before a pixel is marked as water in the output. This can dramatically reduce false-positives from cloud-shadows, and may permit flooding to be observed when clouds move out of the way. But they may also not be as up-to-date as the 1-day product.
Link to 2-day product: https://maps.disasters.nasa.gov/arcgis/home/item.html?id=f7178cdbcc434ff09538999ab5ce8d83
Link to 3-day product: https://maps.disasters.nasa.gov/arcgis/home/item.html?id=b33741d95e924507b75b334f16ec23f1
Please refer to the User Guide for additional details and notes on usage. It is available on the project website:
https://www.earthdata.nasa.gov/global-flood-product