The Surface Water Record and the NASA Experimental Near Real Time Flood Maps

Frequently Asked Questions (FAQ)

Updated: August 20, 2012

1) How can I import these maps into GIS?

The Surface Water Record map web displays are jpg files embedded in html documents. Users can download the jpg files by selecting and right-clicking (in Windows). Then most GIS software requires at least three known coordinate points, manually entered, and knowledge of the map projection (in this case, Plate-Carree). The Near Real Time maps are .png files, which can be similarly downloaded and geocoded.

Beginning in 2012, we are adding Geotiff format files and kmz files to the web pages for each display. Both can be imported directly into many GIS programs.

Some users will instead prefer to access the GIS information from which these maps are constructed. Links are provided on each map sheet web page for connection to the MODIS NRT daily .shp files (commencing in late 2011) and to available collections of pre-2011 information (mainly as MapInfo format files, which can be readily converted into .shp files by most GIS programs if need be).

2) What do the dates of the GIS files and the maps actually indicate?

The Record ingests GIS data from the MODIS NRT processor. Those data are updated daily and are also available to the public ( ). The file dates refer to the last of a two day accumulation, Terra-MODIS and Aqua-MODIS, of four images. We use a processing algorithm that reduces noise from cloud shadows and obscuration from cloud cover: this requires the four image set. A (Julian) date of 2011123 in the GIS file name indicates the data are from satellite passes on the 122nd and 123rd days of 2011.

3) What do the map colors indicate?

The Surface Water Record maps accumulate "daily" GIS files over 10 days in one color: red. This is current surface water and is updated when a new GIS file is incorporated, and the daily file older than 10 days alters to light red. A reference "normal" water layer is shown in dark blue, and superimposed over the red and light red map layers. Each display thus shows current surface water in dark blue and red (no red shows if no flooding above the reference extent). The maps also provide nearly complete spatial coverage (few areas are cloud covered continuously when data is accumulated over 10 days). This approach allows the most recent (today's) information to be incorporated in the mapping if this is urgent, while still providing information about ongoing and very recent flooding that may today be obscured by cloud cover.

Previous surface water, mapped in the current year, now dry land, is shown in light red. Thus, flood water mapped during an ongoing flood event, but not mapped as water during the current 10 day accumulation, is light red. In this way, the waning of a flood event is also mapped as surface water contracts and repeat imaging fails to detect water in these previously flooded areas: red is replaced by light red.

The DFO archival record of prior year flooding, 2000-on, is being added as personnel time permits, and this map layer is light blue. These are areas, also, of known flood hazard, as demonstrated by observation of actual flooding at some time in the recent past.

4) What do these maps show?

The maps illustrate, at maximum MODIS spatial resolution, current surface water, and, by comparison to a MODIS-derived reference water layer, any expansion of water beyond normal limits. Dark blue and red together are normal and flood water. In addition to such current conditions, the Surface Water Record preserves the satellite-observed record of prior flooding. These maps and associated GIS data therefore identify areas of future flood hazard. They are intended as a basic reference atlas, in both graphical and GIS format, providing information about the changing extent of the Earth's surface water over time.

5) What are the Drought versions of the Surface Water Record?

In 2012, we are also adding matching drought displays to each of the Surface Water Record web pages. Current 10-day MODIS water composites do not show water area reductions in the flood displays (the reference water from yr 2000 is shown in blue). However, in many cases, today's surface water is actually contracted compared to that reference water. For the "drought" displays, the most recent surface water is accumulated over 20 days, to ensure complete removal of cloud obsuration. These GIS water outlines, in blue, are then shown superimposed over the reference water layer, coded in yellow. Areas of water contraction are thus shown as yellow outlines around any now-contracted water bodies.

The Surface Water Record provides: 1) a comprehensive global archive, and 2) current status information of surface water extent. It is a basis for our flood inundation mapping, which makes use of the inventory of previously mapped flooding as a context for evalution of on-going events. The complete DFO archive of previously mapped flood events is being added, on a time available basis, and we hope to provide complete ftp access to the very large GIS data set supporting this archived history. Because the Surface Water Record accumulates flood water over a period of 10 days, and thereby removes most cloud cover, its displays may provide the best spatially continuous view of any large flood event. In contrast, the NASA Goddard MODIS NRT maps provide immediate current conditions data, are refreshed each day, and provide end users with information providing access to supporting GIS, kmz, and other files. These two-day composites also show current cloud cover.

Our GSFC/DFO research and applications team solicits your input as to how to make all of these data products and the supporting GIS information more useful to you or your organization.

Prepared by G. R. Brakenridge, University of Colorado, on behalf of the combined DFO/NASA GSFC team, including Albert Kettner, University of Colorado, and Fritz Policelli and Dan Slayback, NASA-GSFC.