The MODIS Near Real-Time Global Flood Product (Collection 61) (BETA) (MCDWD) is processed by NASA's Land, Atmosphere Near real-time Capability for EOS (LANCE). It replaces the previous legacy NASA NRT Global Flood Mapping Product, which was generated from 2011 to 2022 (see below).
The Flood Product is a daily, near-global, ~250 m resolution product showing flood and surface water detected from the twice-daily overpass of the MODIS optical sensors (onboard Terra and Aqua satellites). Although water detection at this scale is relatively straightforward with MODIS’s red and near-infrared bands, cloud and terrain shadows can create false-positives. We employ a multi-observation compositing approach to minimize these errors, generally requiring more than one water observation to mark a product pixel as water (flood or surface water). The product is generated with three compositing periods (1-day, 2-day, and 3-day) because the best product for a given flood event depends on unpredictable factors: the specific area of interest; cloud cover over dates of interest; potential spatial extent of likely flood water; and likely duration of flooding. It is recommended that the user review both the 2- and 3-day products to determine which provides better coverage for a given event. If clouds are not present, or latency is a primary concern, the 1-day product may be appropriate. Currently, only the 2- and 3-day products are viewable in Worldview.
Product generation consists of three key steps:
- Water detection algorithm applied to MODIS observations (Terra and Aqua)
- Compositing of these water detections, over 1, 2, and 3 days, to minimize false positives and fill in observations as clouds move
- Differentiating flood from expected surface water (using MOD44W global water mask).
For more information please see the MODIS NRT Global Flood Product User Guide.
January 12, 2023: Release of Beta 2 version, with User Guide Revision C. This update adds use of a general topographic filter to remove water detections from mountainous areas, greatly reducing the number of terrain shadow false-positives in such areas.
Short Name: MCDWD_L3_NRT
Long Name: MODIS/Aqua+Terra Global Flood Product L3 NRT 250m
Platform: Aqua and Terra, Instrument: MODIS
DOI (Main HDF product):10.5067/MODIS/MCDWD_L3_NRT.061
DOI (GeoTIFFs of each composite):
User Guide: MODIS NRT Global Flood User Guide: Beta 2 version (January 2023)
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Data Access / Download
HDF and GeoTIFF Product Downloads:
- Earthdata LANCE-MODIS page
- LANCE servers: https://nrt3.modaps.eosdis.nasa.gov (primary), or https://nrt4.modaps.eosdis.nasa.gov (backup/secondary)
For HDF files navigate to NRT Data → allData → 61 → MCDWD_L3_NRT
or, go directly to
For GeoTIFFs (one for each of the 4 flood composite products in each HDF file)
navigate to NRT Data → allData → 61 → MCDWD_L3_F<X>_NRT (where X is the composite of interest: 1, 1C, 2, or 3).
Or, go directly to:
Downloading products requires free registration with the Earthdata Login registration system.
For automated and bulk download see the following instructions: https://nrt3.modaps.eosdis.nasa.gov/help/downloads
Users automating downloads by repeatedly polling the servers for new files would be advised to retain the file modification times for downloaded files and compare these to times observed on subsequent polls to determine if a file has been updated and thus should be re-downloaded.
Please see the FAQ, What is the best way to obtain the flood product?
Click on image to view larger version.
Citation and Disclaimer
Please use the following citation: NRT MODIS/Aqua+Terra Global Flood Product MCDWD_L3_NRT distributed from NASA LANCE. Available on-line [https://www.earthdata.nasa.gov/global-flood-product]. DOI: 10.5067/MODIS/MCDWD_L3_NRT.061
For Disclaimer, please see the LANCE Disclaimer.
Legacy NASA NRT Global Flood Mapping Product
From 2012 until April 30, 2022, the precursor legacy NASA NRT Global Flood Mapping Product was being generated. Although very similar, the product had slightly different characteristics (tile naming; pixel size) and output products (including a graphic product and a 14-day product). The legacy product used a “MWP” filename prefix, such as “MWP_2022123_080E030N_3D3OT.tif.” Users transitioning from using the legacy product to the new LANCE MCDWD product should in particular review section 8 of the User Guide for relevant differences.
Frequently Asked Questions
Which product will show me the water for a particular flood event?
It is recommended users read section 9.1 (Use Notes) of the User Guide to better understand the caveats of using the global flood product. Buildings, canopy cover, cloud, and cloud-shadow can obscure flood detection and terrain shadow, cloud-shadow, and volcanic rock can lead to false-positive flood detections. It can also be helpful to examine the corresponding MODIS reflectance imagery in NASA Worldview (https://worldview.earthdata.nasa.gov) to determine the level of cloudiness on the dates of interest, and thus potential sources of false-positives (cloud or terrain shadow) or false-negatives (cloud obscuration, or other data gaps).
Why can I only see the 2- and 3-day products in Worldview?
The 1-day product has more false positive flood detections, so to avoid misleading end users only the 2- and 3-day products are currently available in Worldview.
What caveats should be considered when using the flood mapping product?
Incorrect reference water may result in flood false positives. Figure 1 shows a reservoir in Cambodia formed after the completion of the Lower Sesan II dam in 2017. The reservoir is routinely reported as flood in the product, and yet this is obviously not “flooding” of concern. When the reference water is updated, this will instead be reported as “Surface Water,” and thus shown in cyan/blue in Worldview's display. Note that even then, if the reservoir levels vary seasonally or yearly, some edge areas may be reported as flood if the new reference water layer has not captured its maximal extent.
Volcanic lava fields will often trigger the water detection algorithm because like water, they are optically very dark. The user guide shows an example from the Craters of the Moon area of south-central Idaho.
Cloud and terrain shadow can lead to false positives. Please see section 9 of the User Guide for more information and examples.
Why was a particular flood not detected?
The usual culprit is clouds: If it is simply too cloudy for the MODIS sensors to observe the surface (at its overpass times of roughly 10:30 a.m. and 1:30 p.m., mean local time), during the period when the water is present, the flood will not be reported in this product.
Additionally, if the flood is small in extent (MODIS pixels are approximately 250 m), or if it is under canopy cover (trees will obscure underlying water), or in urban areas (streets may be flooded, but there is likely a larger proportion of non-flooded rooftops), it may not be captured, or captured well.
Also remember the product is based on observations at ~10:30 a.m. local time (Terra overpass) and ~1:30 p.m. (Aqua). Only water present at those times will be detected, ruling out most flash flood events
Can I get a GeoTIFF of the flood product?
Yes, the four individual flood composites in the product HDF file are available as individual GeoTIFF files; please see links in the Data Download section of this guide.
What is the best way to obtain the flood product?
It is recommended that you view the 2- and 3-day flood products in Worldview as this will enable you to rapidly browse through relevant dates and view the product with the corrected reflectance imagery, making clouds and any anomalies in the imagery easier to evaluate.
See Data Download for more information on obtaining product files.
What is the difference between the 1-, 2-, and 3-day product?
The table below highlights the key differences between the various 1-, 2-, and 3-day composite products. Visual examples can be found in the user guide.
|Name||Description (per pixel)|
|Flood 1-Day 250m||Flood product, 1-Day: from current day’s data. (no cloud-shadow masks applied to water detections)|
|Flood 1-Day CS 250m||Flood product, 1-Day: from current day’s data. (cloud-shadow masks applied to water detections)|
|Flood 2-Day 250m||Flood product, 2-Day: from current and previous day’s data.|
|Flood 3-Day 250m||Flood product, 3-Day: from current and previous two day’s data.|
Why are there two 1-day products? Which should I use?
The 1-day CS product has cloud shadow masks applied to the water detections to help remove cloud-shadow false positives. However, these masks can be inaccurate, and thus can remove real water. If you can review the MODIS reflectance imagery (e.g., in Worldview) and confirm there are no clouds over your site, either product is fine, as they should be identical. If you see clouds, then it is recommended you use the “1-day CS” product, but be aware some cloud shadow false-positives may still exist; examine reported flood pixels carefully. In general, it It is recommended that you only use the 1-day product if either: you need the most timely information, or you know there are no potential cloud concerns. If there ARE clouds AND you need the most timely information, it is recommended that you examine both 1-day products to see if either is showing flood water in areas of concern. If either does, then be sure to confirm from the visible imagery (most easily done via Worldview) that the reported flood pixels are not falling on cloud shadows for either the Terra or Aqua observation.
Why does a known reservoir show up as flooded?
The reference water layer tells us where “normal” water is expected to be observed: rivers, lakes, reservoirs, oceans. The current reference water layer is the initial version of the MOD44W product, c2009, and is increasingly out of date (see section 9.1 of the User Guide), leading to errors in the product. To address this, a new reference water layer is planned.
What are the differences between the new LANCE flood product and the legacy flood product?
The LANCE MCDWD 1-, 2-, and 3-day composite products are considered comparable with the legacy product. The significant differences in terms of water detection are an increased number of cloud-shadow false-positives in northern latitudes (mostly 50º-70ºN). User Guide section 4.1 contains more details. We plan to address this in an update later this year.
A practical difference is in distribution format. The LANCE product is delivered in one HDF file per tile, per day (although GeoTIFF files are also available for the flood layers), whereas the legacy product is delivered in individual GeoTIFF files per day and composite period (the MWP product). Tile size (and pixel sizes) are also slightly different. The table below shows how flood product data values have also changed between legacy MWP and LANCE MCDWD. See User Guide section 8 for more information.
Legacy flood product (MWP) data values
LANCE flood product (MCDWD) data values
* Note the legacy product did not have the “recurring flood” label, and although this is planned for the LANCE product, it will not be implemented immediately
What improvements are planned?
Future improvements include: Addressing the larger number of cloud-shadow false-positives in northern latitudes, updating the reference water map, processing and making available historical products back to 2003, adding a new "recurring flood" data value to the product, and incorporating VIIRS imagery to the product. See User Guide section 7 for more information.