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A New Tool for Investigating Coastal-Sediment-Transport Data in Google Earth


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Sample experiment page
Above: Sample experiment page, showing link to a KML file highlighted in yellow. [larger version]

Initial Google Earth display
Above: Initial Google Earth display of the KML file MYRTLEBEACH.kml. [larger version]

Expanded view of co-located files
Above: Expanded view of co-located files, plus information balloon containing metadata from file 7265sc-a.nc. [larger version]

U.S. Geological Survey (USGS) scientists in the Coastal and Marine Geology Program (CMGP) recently implemented a new way of browsing CMGP's sediment-transport data by using the virtual-globe program Google Earth. Ellyn Montgomery and Rich Signell of the USGS Woods Hole Science Center created Keyhole Markup Language (KML) files for CMGP sediment-transport experiments in the Atlantic and Pacific Oceans, the Gulf of Mexico, and the Adriatic Sea. The KML files employ Google Earth placemarks with information balloons to display metadata fields for each file, enabling intuitive investigation of data collected during each experiment. The creators, Montgomery and Signell, anticipate that users will benefit from the convenience of using Google Earth to browse through the data.

The homepage "USGS Woods Hole Science Center Moored Time Series Data" contains links to pages about the sediment-transport experiments, organized by region and experiment. All the experiment pages have a similar structure; in this article, we use the Myrtle Beach experiment as an example. Once the KML files were made, a new section was added to each experiment page that provides a link to the KML file (highlighted in yellow on the reproduced Web page above).

Most browsers launch Google Earth automatically when a KML file is selected. Once launched, Google Earth opens the KML file and zooms in to show the area surrounding the instruments deployed for the experiment (see sample display at right). Yellow icons indicate placemarks associated with data files. The text beside each placemark is the name of the file, whose contents are summarized in a window, or "information balloon," that opens when the placemark is selected. A time slider (semi-opaque horizontal bar at center top of Google Earth views to right) shows the duration of the experiment. The KML file opens with only the beginning of the experiment shown; sliding the bar to the right causes more time to be displayed. The time slider is especially useful with multiyear monitoring programs, for which the user might want to display only those files collected in a particular year.

If several instruments were deployed in one location, the files for some of them will be hidden in the initial Google Earth view. In such cases, clicking on a yellow icon will force an expansion of the items, showing the co-located files connected to the central location by lines. The Google Earth view at right shows such an expanded view of all the files available at a particular location. The expanded view also shows the information balloon displaying metadata for the selected file. The metadata fields listed for all files include position, duration, types of variables collected, water depth, measurement depth, and the URL of the Web page where data are posted. If an entry is missing in the metadata, it will be indicated in the display as "unavailable". The example file contains temperature and conductivity measurements collected at 4.934 meters below the water surface, from which salinity and density (sigma-theta) were computed.

Perhaps the most important metadata field is the URL of the Web page where the data file is provided by our OPeNDAP server. When that URL is clicked, a new Web-browser tab opens to display an OPeNDAP viewer, which allows more detailed browsing of the attributes and data in the file. OPeNDAP is open-source software for data access across the network widely used in the Earth-science community. A DAP-enabled client can use the URL to directly access any of the data identified in the file. This is a tremendous advantage in that a user does not have to download data files in order to use them; the client simply requests the data over the network and uses it as it is supplied.

The KML files that define what is displayed in the Google Earth views were created by using a user-contributed Google Earth toolbox for Matlab, and software that Montgomery and Signell developed to extract metadata from the NetCDF (Network Common Data Format) data files to create an information balloon for each instrument or file. For information on how to acquire the software and adapt it to your data, please contact Ellyn Montgomery at emontgomery@usgs.gov or (508) 457-2356.


Related Web Sites
Woods Hole Science Center Moored Time Series Data
USGS (U.S. Geological Survey)

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Research
cover story:
Water-Quality Monitoring

New Method to Estimate Sea-Ice Thickness

Meetings Airborne Lidar Processing System Workshop

Staff Bill Normark Passes Away

Bill Normark: Tributes

Bill Normark: Ascent of Sea Cliff

Bill Normark: Pisces Dive P5-78

St. Petersburg Office Dedicates New Building

USGS Deputy Director Addresses Downtown Partnership

Publications Coastal-Sediment-Transport Data in Google Earth

June 2008 Publications List


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