Earthdata Forum

Posted: **Thu Mar 05, 2026 1:31 pm America/New_York**

Version: 2/17/2026

Major sections:

Getting Started
Product Usage
Worldview and FLOOD viewer
Support and Mailing Lists

Getting started

What is the NRT Flood Product?

NASA's Land, Atmosphere Near real-time Capability for Earth observation (LANCE) provides two global daily ~250 m resolution near-real time (NRT) flood products:
- MODIS product (MCDWD) generated from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument imagery on the Terra and Aqua platforms.
- VIIRS product (VCDWD) generated from the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument imagery on the NOAA-20 (JPSS-1) and NOAA-21 (JPSS-2) platforms.
The MODIS product has been in near real-time production since early 2021 (although the archive has recently been generated back through 2003). Prior to this, a legacy product (“MWP”) was generated from 2011 through 2022. The VIIRS product was released in April 2025. Apart from the difference in input data source (MODIS or VIIRS), the products are otherwise the same in algorithm and product formats. With the Terra and Aqua satellites reaching end of life, the VIIRS product was developed to replace the MODIS product.

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

The product relies on optical satellite imagery taken at specific times of day to observe floods. And as this is optical imagery, it cannot see through clouds. Thus, detectible floods must be on-the-ground when we have cloud-free observations. (see FAQ “How often are the data acquired?”)
The flood must also be large enough to be detectible by 250 m (MODIS) or 375 m (VIIRS) pixels. Flash-floods constrained to relatively narrow river valleys may not be dimensionally large enough to be reliable detected, apart from likely not being on-the-ground when the satellites make observations.
The product has 3 different composites: 1-Day, 2-Day, and 3-Day. These indicate how much data was used to generate the product: 1-day = only the current day’s data.
It is recommended users read section 7.1 (Usage Notes) of the User Guide to better understand usage and caveats.
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 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).

What is the best way to view or download the flood product?

In Worldview, users can rapidly browse the 2- and 3-day flood products, and can also view relevant corrected reflectance imagery, which can assist in evaluating any anomalies from cloud shadows or data coverage.
In the FLOOD viewer, users can view the 1-day product in addition to the 2- and 3-day products. The 1-day product is only available in the ADVANCED mode. This is intended for more experienced users, as it may contain false-positive (particularly from cloud-shadows). The FLOOD viewer also hosts other flood products, so users can easily compare them.
Flood imagery can be streamed from GIBS (Global Imagery Browse Services) into users GIS or other applications. This is the same imagery that is displayed in Worldview or the FLOOD viewer; please note these are graphic imagery layers, and not data layers.
For recent data (within past ~week) flood products can be downloaded from the LANCE NRT servers. The primary server is [urlhttps://nrt3.modaps.eosdis.nasa.gov/]nrt3.modaps.eosdis.nasa.gov[/url]. A free NASA Earthdata User Account is necessary to download data.
For MODIS flood file product downloads, navigate as follows from the nrt3 homepage, Browse NRT Data -> allData -> 61 -> MCDWD_<Product>, where <Product> is L3_NRT for the full HDF product files (with all layers), and L3_F1_NRT, L3_F1C_NRT, L3_F2_NRT, and L3_F3_NRT for geotiff files (1-day, 1-day CS, 2-day, and 3-day, respectively).
For VIIRS flood product file downloads, navigate as follows: Browse NRT Data -> allData -> 5200 -> VCDWD_<Product>, where <Product> is L3_NRT for the full HDF product files (with all layers) for days prior to the current day, and geotiff files in F1, F1C, F2, and F3 directories as above for MODIS. Partial files for the current date (eg including data up to the time of access) are found in VCDWDG_L3_NRT.
Please see the homepage or User Guide for additional details.

Where is the User Guide?

Here is the current user guide, and also under “Summary” on the product homepage

How often are the data acquired?

Both the MODIS and VIIRS products use daily observations from two satellites: MODIS from the Terra and Aqua satellites, and VIIRS from the NOAA-20 and NOAA-21 satellites. Thus each product has two daily observations.
For MODIS, these observations are more widely spaced in time, with Terra historically observing at about 10:30 AM local time, and Aqua observing at about 1:30 PM. The Terra and Aqua observation times have slipped since 2020, and 2022 (respectively), to earlier and later times, as the spacecraft have run out of fuel for orbit maintenance maneuvers. As of late 2025, they are overpassing at ~9:00 AM (Terra) and 3:00 PM (Aqua).
For VIIRS, the observations are at about 1:30 PM (NOAA-21) and 2:20 PM (NOAA-20).
The observation times are mean equatorial crossing times: the local solar time when the satellite crosses the equator. These satellites take ~100 minutes to orbit the earth, so approximately 50 minutes are spent on the daylit side of the earth, traveling from pole to pole. Thus, the local time at overpass may be offset from the nominal equatorial times by up to 25 minutes (depending on latitude).
For an area observed in a given orbit, local solar time does not necessarily equal local clock time, as the latter depends on politically drawn time-zones, etc.

What time does the satellite overpass?

Please see above FAQ: How often are the data acquired?

What dates are the flood product available for?

The product has been reprocessed back to 2003. Those historical data will be made available shortly; please check the User Guide for links to this, and/or subscribe to the mailing list to receive product announcements (Support section of homepage has info on mailing list)
Worldview and the FLOOD viewer only display the MODIS product from early 2021, and the VIIRS product from June 2025 (when it was released), as those viewers pull from the GIBS imagery layers (a separate step and product than the raw data files). We hope to populate GIBS with the MODIS historical flood product back to 2003 in the future.

Why was a particular flood not detected?

Typically: clouds. If an area is simply too cloudy for the sensors to observe the surface during a flooding event at the satellite overpass times, the flood will not be reported in this product.
Alternatively, the flood may not have been present at the satellite observation times. And in particular, flash floods that may only be on the ground for a few hours are not usually observed. Please also see FAQ above on observation times (“How often are the data acquired?”)
And, 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.
Section 7.1 (Usage Notes) of the User Guide also provides more details.

What is MODIS vs VIIRS?

MODIS is the MODerate resolution Imaging Spectrometer, a NASA instrument on board two separate spacecraft (Terra, launched 1999, and Aqua, launched 2002). Each instrument provides daily multispectral near-global imagery of the earth.
VIIRS is the Visible Infrared Imaging Radiometer Suite, a joint NOAA-NASA instrument that was developed from MODIS, and is currently on three spacecraft: S-NPP (launched 2011), NOAA-20 (aka JPSS-1, launched 2017), and NOAA-21 (aka JPSS-2, launched 2022). The imagery from NOAA-20 and NOAA-21 are used in the VIIRS flood product.
The highest resolution MODIS bands (which are the primary component of the MODIS flood product) are 250 m resolution. While for VIIRS, the principal bands are 375 m resolution.

How can I ingest the product into a GIS?

There are 3 main options: (1) download the HDF product file, extract the layers of interest to geotiffs, and bring those into a GIS (see User Guide section 7.3 for sample GDAL code to extract layers); (2) download the geotiff product file (only available for NRT products within last ~1 week), and bring into GIS; (3) use the GIBS APIs to view the product, without having to deal with downloading files.
- See FAQ above (“What is the best way to view or download the flood product?”) for details on downloading.
- The GIBS documentation has information and examples on how to add the API to QGIS, ArcGIS Pro, ArcMap, ArcGIS Online, and Google Earth.

Can I get a geotiff?

Yes. Please see FAQ “What is the best way to view or download the flood product?” or the User Guide for more details.

Product Usage

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.

Name	Description
1-Day	Flood product, 1-Day: data from the current day.
1-Day CS	Flood product, 1-Day: data from the current day, with cloud-shadow masks applied to water detections.
2-Day	Flood product, 2-Day: data from the current and previous day.
3-Day	Flood product, 3-Day: data from current and two previous days

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 or the FLOOD viewer) to determine which provides the most useful information.

What are the differences between the new LANCE flood product (MCDWD and VCDWD) and the legacy flood product (MWP)?

The legacy flood product (MODIS product delivered in MWP files) was generated from 2011 through 2022, while the updated LANCE product was released in March 2021.
In 2025, the MCDWD product was reprocessed from 2003 through 2024 to provide an updated and complete archive, back to the start of the MODIS/Aqua era (Aqua was launched in 2002).
The LANCE MCDWD products are considered comparable with the legacy product, although the MCDWD has added some additional features: a dynamically updated reference water layer, an additional topographic mask (HAND: Height Above Nearest Drainage), and the recurring flood class. The compositing is also somewhat different in the LANCE product, as this product incorporates all overlapping swath observations. Section 4 (Product Evaluation) in the User Guide discusses differences observed between the products, and Appendix 2 provides additional details.

What are the major limitations of the product?

Clouds may block clear view of the surface, and thus flood detection. The 1, 2, and 3-day composites are an attempt to minimize this limitation, but with complete cloud cover over the flood period of interest, the product will be unable to detect flooding.
Flash floods may not be present long enough to be observed, cloud-free, and so are often not detected.
Floods of small spatial extent, which is often true of flash flooding along river valleys, may also not be captured, if the open water is not on the order of the resolution of the sensors (250 m for MODIS product, 375 m for VIIRS).
Cloud shadows may cause false-positives. The multi-day compositing will minimize but not entirely eliminate this, especially for the 1-day composite.
Terrain shadow may cause false-positives in mountains, especially in winter. We screen out most of these, but some may persist. Please see section 7 of the User Guide for more information and examples on cloud shadow and terrain shadow false-positives.
The reference water map, which is used to separate out normal or expected water from flood, may have inaccuracies in some areas, causing flood to be reported as surface water, or normal surface water to be reported as flood. This is particularly a problem with newly constructed reservoirs – these will likely appear as “flood” for 2-3 years after filling,
Volcanic lava fields will often trigger the water detection algorithm because like water, they are optically very dark. The User Guide (section 7) shows an example from the Craters of the Moon area of south-central Idaho.
Dense vegetation can obscure surface water, making floods difficult to detect. Additionally, sun glint over open water or reflective surfaces can distort reflectance measurements, leading to misclassification in the flood product (typically with water not being detected).

Can this be used as an alert, to identify new/emerging floods?

Potentially, yes. However, false-positives do occur, so any alert system will need to take this into consideration. We are considering methods to filter the data for this purpose and hope to release some form of alert in the future.

Are there false flood detections?

Yes, largely due to cloud-shadow false-positives, and terrain-shadow false-positives. See above FAQ “What are the major limitations of the product?” for more details.
To minimize mis-interpretation, viewing the source imagery should help discern whether a reported flood is real or an artifact from cloud or terrain shadows. This is most easily done in Worldview, or the FLOOD viewer, where the source datasets (MODIS/Terra, MODIS/Aqua, VIIRS/NOAA-20, and VIIRS/NOAA-21) are all easily viewable. If flood pixels are falling under areas of dark shadows on any of the days used in the composite, they should be evaluated carefully.

Are there missed flood detections (false-negatives)?

Yes, largely due to cloud-obscuration of the surface, or small flood extent (MODIS pixels are 250 m, and VIIRS are 375 m), or missed timing (if the flood is not present when the satellite observations occur: see “How often are the data acquired?”).

Can flood be detected below forest canopy?

Generally no, unless there are breaks in the canopy allowing the water to be observed by the satellite.

Can flood be detected in urban areas?

Generally no, unless there are areas of open water at or above the size of the observation pixels (250 m for MODIS, 375 for VIIRS). Typically dry building roofs and tree tops outweigh the signal from flooded streets, unless these are wider boulevards or highways.

Can flash flooding be detected?

Generally no, because typically the water is only present for a short period. If the satellite has a clear observation during that window, then the product may detect the flood. But fast-moving flash floods are also often constrained to relatively narrow spatial extents, along river valleys or canyons, and so may present only a thin ribbon of water, which will be difficult to detect with the sensors 250 m (MODIS) or 375 m (VIIRS) pixels.

Can it detect glacier outburst flood events?

Similar to issues with detection of flash flooding (see above FAQ “Can flash flooding be detected?”), this is generally unlikely. Mountainous areas where such floods occur may also have issues with terrain shadows. However, if the flood waters then flood a flatter downstream valleys or plains, and persist for a day or more, those components of the flood are more likely to be detected.

Why is a known reservoir being reported as flood?

The reference water layer tells us where “normal” water is expected to be observed: rivers, lakes, reservoirs, oceans. The 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 to 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.

Does the HAND topographic mask affect urban areas?

The HAND (Height Above Nearest Drainage) mask is designed to remove false-positives (from terrain shadows in areas with substantive topographic relief) from the product. We have not specifically looked at how it performs over urban areas. The DEM dataset used to generate HAND is the Copernicus 90-m DEM product (GLO-90). To the extent this captures buildings in urban areas, the HAND mask may impact those areas.
We plan to distribute the HAND mask, and make it viewable in the FLOOD viewer, so users can directly determine where it may be impacting potential flood observations. Our implementation of the HAND mask is attempting to identify areas where flood water would be very unlikely to remain for any length of time, due to sufficient likely nearby drainage. That said, a flash flood might also occur in such an area masked by HAND, but the water will not persist for that very reason: the drainage potential of nearby streams and channels.

FLOOD viewer and Worldview web-based tools

What is the difference between using Worldview or the FLOOD viewer to view the product?

The NASA Worldview tool is a general purpose web-based viewer for over 1000 NASA data layers; the LANCE Flood products are just a few of the included layers that users may view. Users familiar with Worldview and its many capabilities may prefer to use this avenue. The main disadvantage is that the 1-day composite flood products are not available in Worldview; due to the higher likelihood of false-positives, and the broader reach of the Worldview tool, we limit those layers to the Advanced mode of the FLOOD viewer. Thus we hope only users cognizant of their limitations will explore them.
The FLOOD viewer is purpose-built for viewing the LANCE flood products and related layers, including flood products from other organizations. All flood products have strengths and weaknesses, and thus being able to directly compare them in context, along with the source imagery, can be quite helpful to selecting the best product for a given event. Currently the ADVANCED mode of the FLOOD viewer includes the following flood-related products:
- NOAA-GMU VIIRS-based flood product (1- and 5-day versions).
- NASA OPERA Dynamic Surface Water Extent products (both the HLS (optical) and Sentinel-1 (SAR) products).
- NASA OPERA Radiometric Terrain Corrected SAR backscatter from Sentinel-1 (RTC-S1). It provides terrain-corrected radar images with a 30-meter pixel spacing and a temporal resolution of a few days to weeks on average, depending on the location. This can provide valuable context for potential flooding.
We hope to bring other flood layers into this viewer over time.

.
Both viewers pull NASA flood products from the GIBS imagery layers that are generated from the products; these same layers can be pulled into user’s GIS or other applications via the GIBS API.

Can I download the product from the FLOOD viewer or Worldview?

The viewers do not provide the ability to directly download the raw flood product files at this time. However, Worldview will redirect users to an Earthdata Search page with relevant results if the data are recent and still available on the LANCE NRT servers; generally data remain available only for about 1 week.
Both viewers display flood layers served by GIBS, which are graphic image layers generated from the raw data products. Users can also access the GIBS layers using the GIBS API, and pull them into ArcGIS, QGIS, or other applications.

How often are the flood layers displayed in Worldview, or the FLOOD viewer, updated?

The flood product available in Worldview and the FLOOD viewer is updated whenever new data has been received and made available as a GIBS (Global Imagery Browse Services) layer.
The satellites providing the source imagery (Terra and Aqua for MODIS, and NOAA-20 and NOAA-21 for VIIRS) take 100 minutes to orbit the earth, so the vertical swaths that appear in Worldview (or the FLOOD viewer) take 50 minutes to acquire, and then a 50 minute gap (while the satellite is on the other side of the planet) before the next swath will begin to appear. Data is processed in much smaller chunks than the full swath, so the swath will be displayed in chunks, as that data is received. Note this is much easier to observe with the Corrected Reflectance imagery (loaded as default layers in both Worldview and the FLOOD Viewer) because this is continuous visible imagery, while the flood product’s swaths can be less obvious due to cloud cover creating large gaps in coverage.
As NRT (near real-time) products, the product files themselves should always be available within 3 hours of the satellite overpass. In practice, it is usually sooner. Generation of the GIBS layers (which is what the FLOOD viewer and Worldview are actually displaying) requires an additional 1-2 hours. So there may be a total delay of up to 5 hours from observation, before the flood product is visible (or updated with latest observations) in the Worldview or FLOOD viewers.
In general, if the Corrected Reflectance imagery layers are available for the area of interest, for the current day, then the flood product should also be available, or should appear soon.
For the latest update, users can check the download directories for the most recent product files; these will appear as much as 1-2 hours before the displays in the FLOOD viewer or Worldview are updated.

Support & mailing lists

Where is the User Guide?

Here is the current User Guide, which you can also find under “Summary” on the product homepage.

Who do I contact for support with the product?

Please email earthdata-support@nasa.gov

How can I stay informed of updates, announcements, data issues, and scheduled maintenance?

For flood-specific announcements, please sign up for our low-volume mailing list by sending an email to: floodmap-join@lists.nasa.gov (no text or subject required).
For alerts about LANCE production system issues, please sign up for the LANCE-MODIS mailing list by sending an email to: lance-modis-join@lists.nasa.gov (no subject or body text is required).
Keep an eye on the “Updates” section of the product homepage.

Where can I go to interact with other users and NASA subject matter experts on this product and related topics?

Try the NASA Earthdata Forum, for questions and answers about this product, and other related datasets.

Earthdata Forum

Flood Product FAQs

Flood Product FAQs