MODIS NRT Global Flood Product


Flood in the Lower Mekong region of Cambodia

Short Name: MCDWD_L3_NRT

Long Name: MODIS/Aqua+Terra Global Flood Product L3 NRT 250m

DOI:10.5067/MODIS/MCDWD_L3_NRT.061 (HDF file)

DOIs GeoTIFFs:

10.5067/MODIS/MCDWD_L3_F1_NRT.061
10.5067/MODIS/MCDWD_L3_F1C_NRT.061
10.5067/MODIS/MCDWD_L3_F2_NRT.061
10.5067/MODIS/MCDWD_L3_F3_NRT.061

Platform: Aqua and Terra, Instrument: MODIS

Links to view the data in Worldview: https://go.nasa.gov/39L5hX6.

Data download

Data download

Data can be downloaded from the Earthdata LANCE-MODIS page or directly from the LANCE servers: https://nrt3.modaps.eosdis.nasa.gov 

If the NRT3 server is unavailable, users can download the data from the secondary server https://nrt4.modaps.eosdis.nasa.gov 

For 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

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:

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

Downloading products requires free registration with the Earthdata Login registration system.

For automated and bulk download see the following for 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 note: At the initial release in March 2021, the nrt download sites are only populated with product files at the end of the day. In the near future (expected in July 2021), the files will be updated throughout the day, as each incoming swath granule is processed. 

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

Summary

Summary

The MODIS Near Real-Time Global Flood Product (Collection 61) (BETA) is processed by NASA's Land, Atmosphere Near real-time Capability for EOS (LANCE) and will replace NASA's legacy Goddard Global Flood Product. To enable users to transition smoothly, there will be an overlap through to August 31, 2021) where the LANCE and legacy products will be available.

The flood product has 3 compositing periods (1-day, 2-day, and 3-day) because the best product for a given flood event depends on four unpredictable factors: the specific area of interest; cloud cover; spatial extent of likely flood water; and likely duration of flooding. It is recommend that the user view both the 2 and 3 day products unless latency is a primary concern. Ultimately determining which product provides the best information for a particular event of interest will be up to the user.

Product generation consists of three key steps:

  1. water detection algorithm applied to each MODIS observation;
  2. compositing of these water detections, over time, to reduce errors and more rigorously identify water; and
  3. differentiating flood from expected surface water.

The compositing step helps limit false-positives.

For more information please see the MODIS NRT Global Flood Product User Guide.

Citation and Disclaimer

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://earthdata.nasa.gov/earth-observation-data/near-real-time/mcdwd-nrt]. DOI: 10.5067/MODIS/MCDWD_L3_NRT.061

For Disclaimer, please see the LANCE Disclaimer.

Frequently Asked Questions

Frequently Asked Questions

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

It is recommended users read section 9.1 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. Having understood the caveats of using the data, it is recommended users then examine the corresponding MODIS reflectance imagery in the Worldview (https://worldview.earthdata.nasa.gov), to determine the level of cloudiness on the dates of interest.

Why can I only see the 2 and 3-day products in Worldview?

The one 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?

Flood mapping using optical satellite data is challenging and users should be aware of the following:

Incorrect reference water may result in flooding 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 “flooding” is not of concern. When the reference water is updated, this will instead be reported as “Surface Water”, and thus shown in cyan/blue in Worldview. 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.

Lower Sesan II dam identified as flood

Screenshot from NASA's Worldview showing the reservoir behind the Lower Sesan II dam in Cambodia. Surface water is shown in cyan/blue and flood water in red. In this example, the reservoir is mis-labelled as flood (in red) even though it is a known body of water. This image is from the 2-day product shown for 24 November 2020.

 

Volcanic lava fields will often trigger the water detection algorithm because like water, they are optically very dark, and thus can be reported as 'flood'. The user guide shows an example from the Craters of the Moon area of south-central Idaho.

Persistent cloud and terrain shadow false-positives can lead to false positives. See section 9.2 of the user guide for more information.

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 AM and 1:30 PM local times), 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 square), or if it is under canopy cover (tree tops not being detected as water), or in urban areas (streets may be flooded, but there is likely a larger proportion of non-flooded rooftops providing signal to the satellite), it may not be captured, or captured well.

Can I still access the legacy flood product?

Yes - for now you can still access the NASA's legacy Global Flood Product but the LANCE flood product will replace it in the near future. To enable users to transition smoothly, there will be an overlap of approximately 3 months (through to August 31, 2021) where the LANCE and legacy products will be available.

Can I get a GeoTIFF of the flood product?

Yes, the 4 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 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: (1) you need the most timely information, or (2) you know there are no potential cloud concerns (by having reviewed the optical imagery in the Worldview interface (https://worldview.earthdata.nasa.gov), for example). 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 50-70N). 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 will also be available for the flood layers), whereas the legacy product is delivered in individual GeoTIFF files per day (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.

Description

Legacy flood product (MWP) data values

LANCE flood product (MCDWD) data values

No Water

1

0

Surface Water

2

1

Recurring Flood*

NA

2

Flood

3

3

Insufficient Data

0

255

* 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 terrain shadow masks to be more effective- update the reference water map, and adding a new "recurring flood" data value to the product. See User Guide section 7 for more information. 

Page Last Updated: Jun 29, 2021 at 9:00 AM EDT