NRT Global Flood Products

Summary

The MODIS and VIIRS products use the same overall algorithm and processing, with a few differences.

• Resolution: MODIS source imagery is nominally at ~250 m resolution for the primary red and near-infrared bands used). VIIRS source imagery is 375 m resolution for those bands. To simplify adapting the code to work for VIIRS, the incoming VIIRS data is resampled to the same ~250 m lat/lon grid as the MODIS product. But because the VIIRS observations are fundamentally at lower resolution, it is expected to do less well at detecting small bodies of water.
• Observation times: Until 2022, the nominal equatorial overpass times for MODIS were 10:30 a.m. for Terra, and 1:30 p.m. for Aqua.  As the satellites reach end-of-life and fuel supplies are exhausted, these have drifted to ~9:30 a.m. and ~2:30 p.m., as of March 2025, and will continue to drift. All VIIRS instruments only provide afternoon observations, nominally ~1:30 p.m., although currently NOAA-20 and NOAA-21 are separated by about 50 minutes. The 50 minute separation was found to be sufficient to improve the product, compared to using data from only a single VIIRS instrument. But the VIIRS product is expected to be of somewhat lower quality than the MODIS product due to loss of the generally less cloudy morning observations provided by Terra. Future updates to the User Guide will contain a more detailed analysis of this impact, as both products are generated in tandem and the results analyzed.
• Current day files: The MODIS MCDWD product files are updated in-place as new observations are received and processed throughout the day. The user must check timestamps on the files to determine if they have been updated. For the VIIRS product, there is a separate hourly cumulative product generated for the current day: VCDWDG. The filenames have an additional element indicating the hour of production. For example: VCDWDG_L3_NRT.A2025063.2000.h08v06.002.h5 was generated at 20:00 (GMT). For a given tile, there will likely be 2 to 4 such files generated over the course of a day, as additional observations intersecting that tile are received. Thus, a user interested in the product for the current day should select the file with the highest hourly timestamp. It will incorporate all data received to that point in the day. The file generated with what would be a 2300 timestamp contains all the data received for the day, so is generated as the final VCDWD product, not as VCDWDG (and so will be located in the VCDWD download directories).  
• HDF format: The MODIS product is generated in EOS-HDF4 (with .hdf file extensions), while the VIIRS product is in EOS-HDF5 (with .h5 extension).

Updates

March 26, 2025: VIIRS product Beta 1 published. (User guide Revision E). The final calibration for the VIIRS surface reflectance product for NOAA-21 is still under development. In this Beta 1 release, we are using preliminary calibration.

April 16, 2024: MODIS product Release 1 version published (User Guide Revision D). The major components of this update include: adjusting the compositing threshold to minimize false-positives at higher latitudes; updating the reference water layer to address issues with new water bodies (mostly reservoirs filled after 2003); and expanding production to additional tiles. Note also the layer names in the HDF product file have changed, which may impact automated processing routines. 

January 12, 2023: MODIS product Beta 2 version (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.

March 5, 2021: Initial MCDWD beta release (User Guide Revision A)

MODIS Product Details

VIIRS Product Details

User Guide: MODIS/VIIRS NRT Global Flood User Guide: revision E, March 2025

Support

• Contact Earthdata Support
• To be notified of updates, please email floodmap-join@lists.nasa.gov (no text or subject required)
  • To unsubscribe:e-mail floodmap-leave@lists.nasa.gov (no subject or text is required)
• Main POC - Dan Slayback, NASA Goddard Space Flight Center

Data Access/Download

View the data in Worldview: https://go.nasa.gov/3OiKtYB and view the "Assessing Floodwaters" Tour Story in Worldview: https://go.nasa.gov/3kglhIJ 

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) 

MODIS HDF Files

For MODIS HDF files navigate to: NRT Data → allData → 61 → MCDWD_L3_NRT 
or, go directly to https://nrt3.modaps.eosdis.nasa.gov/archive/allData/61/MCDWD_L3_NRT

VIIRS HDF Files

For VIIRS HDF files navigate to NRT Data → all Data → 5200 → VCDWD_L3_NRT (or VCDWDG_L3_NRT for hourly)
or, go directly to
https://nrt3.modaps.eosdis.nasa.gov/archive/allData/5200/VCDWD_L3_NRT

MODIS GeoTIFFs

For MODIS 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:

https://nrt3.modaps.eosdis.nasa.gov/archive/allData/61/MCDWD_L3_F1C_NRT
https://nrt3.modaps.eosdis.nasa.gov/archive/allData/61/MCDWD_L3_F1_NRT
https://nrt3.modaps.eosdis.nasa.gov/archive/allData/61/MCDWD_L3_F2_NRT
https://nrt3.modaps.eosdis.nasa.gov/archive/allData/61/MCDWD_L3_F3_NRT

VIIRS GeoTIFFs

VIIRS GeoTIFFs are not yet available, but will follow above conventions when available.

Product downloads require 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

Please see the FAQ below: What is the best way to obtain the flood product?

Tile Map

Citations and Disclaimer

Citation for the MODIS product: NRT MODIS/Aqua+Terra Global Flood Product MCDWD_L3_NRT distributed from NASA LANCE. Available online [https://www.earthdata.nasa.gov/global-flood-product]. doi:10.5067/MODIS/MCDWD_L3_NRT.061

Citation for the daily VIIRS product: VIIRS/JPSS1+JPSS2 Daily L3 Global Flood Composite 250m Linear Lat Lon Grid NRT (VCDWD_L3_NRT) distributed from NASA LANCE. Available online [https://www.earthdata.nasa.gov/global-flood-product]. doi:10.5067/VIIRS/VCDWD_L3_NRT.002

Citation for the hourly VIIRS product: VIIRS/JPSS1+JPSS2 Hourly Cumulative L3 Global Flood Composite 250m Linear Lat Lon Grid - NRT (VCDWDG_L3_NRT) distributed from NASA LANCE. Available on-line [https://www.earthdata.nasa.gov/global-flood-product]. doi:10.5067/VIIRS/VCDWDG_L3_NRT.002

For Disclaimer, please see the LANCE Disclaimer on our Data Use Policy page.

Legacy NASA NRT Global Flood Mapping Product

From 2012 through 2022, the precursor legacy NASA NRT Global Flood Mapping Product was generated. Although 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 the User Guide Appendix for relevant differences.

Frequently Asked Questions

Which product will show me the water for a particular flood event?

It is recommended users read section 7.1 (Usage Notes) of the User Guide to better understand the caveats for use of the 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 is also helpful to examine the corresponding reflectance imagery in NASA Worldview 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 misinterpretation, only the 2- and 3-day products are currently available in Worldview. The 1-day is available in the full HDF product file, and as separate GeoTIFF files.

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. Prior to Release 1, this reservoir was routinely reported as flood. With the updated reference water layer (see User Guide section 3.5) this is no longer occurring (the reservoir is now reported as “surface water”), but similar situations will occur with other newly constructed reservoirs, until they have been filled for ~3 years.  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 8 of the User Guide for more information and examples.

Why was a particular flood not detected?

The usual culprit is clouds: If an area is simply too cloudy for the sensors to observe the surface during a flooding event at its overpass times, the flood will not be reported in this product. Or if the flood water was not present at the overpass times (e.g., flash floods that may be on the ground for only a few hours), it may not be detected. For the MODIS product, overpass times are currently ~9:30 a.m. (Terra) and 2:30 p.m. (Aqua) (mean local time). For the VIIRS product, they are ~1:30 p.m. (NOAA-21) and ~2:20 p.m. (NOAA-20).

Additionally, if the flood is small in extent (MODIS pixels are approximately 250 m, and VIIRS ~375 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.

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 users view the 2- and 3-day flood products in Worldview as this allows rapid browsing of relevant dates, and the user can also view the relevant 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 products?

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.

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 may potentially remove real water, or they may not remove all cloud shadow. In general, it is recommended that the 1-day product is used only if either: the most timely information is needed, or there are no potential cloud concerns. If there are clouds and the most timely information is required, then both 1-day products should be evaluated along with the visible imagery (most easily done via Worldview) to determine which provides the most useful information.

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 generated from the previous 5 years of the annual MOD44W product (see section 3.5 of the User Guide). After a new reservoir is constructed, it is expected it will continue to show up as “flood” in the product for up to 3 years. If the reservoir is more than 3 years old but continues to show up as “flood”, this is likely because the water levels vary enough seasonally (or from year to year) to prevent it from being identified in the MOD44W surface water product. Users could use a custom water mask to reclassify reported flood and surface water pixels accordingly.

What are the differences between the new LANCE flood product and the legacy flood product?

The LANCE MODIS MCDWD 1-, 2-, and 3-day composite products are considered comparable with the legacy product, although the MCDWD has some additional features. Most notably, as of Release 1, the reference water layer has been updated, which greatly reduces the number of falsely reported floods from new reservoirs. The MCDWD also has an additional topographic mask (HAND) applied, which greatly reduces terrain shadow false-positives in mountainous areas (see User Guide section 3.4.2). The flood layer’s pixel values also changed – see the table below. See the User Guide for more information. 

*Note the legacy product did not have the “recurring flood” label, and although this is planned for the LANCE product, it has not yet been implemented.

What improvements are planned?

Future improvements include: adding a new "recurring flood" output class to the product (to discriminate between regularly occurring flooding and unusual flooding). See the User Guide for more information.