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From the Ground Up: Elevation for the Nation, Part 2

July 23, 2008
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This first digital elevation model from the Shuttle Radar Topography Mission (SRTM) shows an area near White Sands, N.M., with coordinates centered at 106 degrees, 47 minutes, 28.8 seconds west longitude and 32 degrees, 58 minutes, 55.5 seconds north latitude. Made available by DLR and NASA.

In my April column, I reported on the push for a new nationwide high-accuracy, high-resolution elevation data set known as “Elevation for the Nation.”

I gave a brief review of the results from the first meeting held last year on this effort. This month, I will report on the second National LiDAR Strategy Meeting held May 21-22 at the headquarters of the U.S. Geological Survey (USGS) in Reston, Va.

In all, 180 professionals were registered for the two-day meeting. Attendees included federal, state and local government officials; surveying and mapping professionals; hardware and software providers; professional organizations; and members of academia. The federal government attendees read like a Who’s Who of agencies: USGS, National Geodetic Survey (NGS), National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Agriculture, U.S. Census Bureau, Federal Aviation Administration (FAA), NASA, National Geospatial-Intelligence Agency (NGA) and U.S. Army Corps of Engineers (USACE).

The Big Picture

Mark Myers, director of the USGS, provided the opening remarks as part of the event’s first session, which was titled “The Big Picture.” He provided a detailed history of topographic mapping in the United States, discussed the involvement of the USGS and illustrated how elevation data plays a vital role in helping our country address a vast array of complex environmental problems.

Myers defined a topographic map as “a map in which elevation is the essential distinguishing quality.” He noted that John Wesley Powell, the second USGS director who served from 1881 to 1894, introduced topographic mapping of the country for use in supporting geologic mapping. In my April column, I provided a detailed timeline of USGS’ involvement in topographic mapping. I won’t repeat that in its entirety here, but it is significant to note that USGS published the first digital elevation model (DEM) in 1975, completed nationwide coverage of the 1: 24,000 topographic maps that are now commonplace in most surveying and mapping offices in 1991, and finished the seamless National Elevation Dataset (NED) in 1997.

The currency of the NED, however, is a major concern. Myers presented a very telling graphic depicting the age of USGS topographic maps found in the NED from the 1940s to the 2000s (the 1970s produced the greatest number of maps). Today, the average age of data in the NED is a little more than 35 years. I am sure you recognize that many areas of our country have seen significant change in that time frame.

A new nationwide effort to capture elevation data was discussed in the National Research Council’s 2007 report titled “Elevation Data for Floodplain Mapping.” The report recognized the need for a varying elevation accuracy and posting depending on terrain and land-cover types and recommended the use of LiDAR technology as the primary means of elevation capture. That recommendation was for a 2-foot contour equivalent for most of the nation with a 4-foot contour equivalent in mountainous terrain and a 1-foot contour equivalent in very flat coastal or inland flood plains.

Approximately 11 percent of the country, including Alaska, has near zero slope, resulting in classification of these areas as high-risk zones. Myers indicated that existing data in the NED is five times older than what is needed to provide reliable information for related analysis. “There is a strong need for consistent, accurate, high-resolution elevation data by state, federal and local users,” he added.

Myers also discussed the Shuttle Radar Topo-graphy Mission (SRTM) and its importance in bringing together a highly accurate, globally consistent topographic map of the Earth’s surface. The SRTM data was collected during an 11-day shuttle mission in February 2000 as a cooperative effort of NASA and the NGA (formerly the National Imagery and Mapping Agency, or NIMA). Data for 99.97 percent of the Earth’s landmass between -57 and +60 degrees latitude was collected during that mission. While this coverage is impressive, the accuracy and post spacing of this data is considerably limiting for many applications. Myers identified the absolute height measurement accuracy for the SRTM data as around 9 meters with a post spacing of 30 meters.

Supporting Perspectives

Following the “Big Picture” were subsequent sessions that were equally important and informative. These included “Data Perspectives,” “The Coastal Perspective,” “The Applications Perspective,” “The High-altitude Perspective” and “Decision Making and Decision Support.”

Jason Stoker, also with the USGS, provided a brief synopsis of the first National LiDAR Strategy Meeting and reviewed 20 questions generated at that first conference, including: “How do we prioritize areas?” “How do we acquire funding?” “Would buy-ins for upgrades be an option?”

During the “Data Perspectives” session, a number of presenters discussed the challenges and successes of various statewide LiDAR collection efforts, including those in Oregon, Pennsylvania and Ohio.

The presentations for both days were very good. Of course, no meeting centered around a nationwide initiative would be complete without a discussion on accuracy assessment, QA/QC workflows, and data formats, storage and dissemination.

An Invaluable Data Set

Several short-term steps critical to the success of a national LiDAR initiative face the USGS. The agency is currently working toward the formation of an interagency national LiDAR working group and envisions including the national LiDAR program startup in the FY 2010 budget.

Creation of a new nationwide elevation data set is a major undertaking. But the number of commercial LiDAR sensors in place in the U.S. private sector provides the production abilities for large-area, high-resolution elevation collection. Moreover, the number of commercial sensors will grow by 2010 as will the capabilities of the individual sensors. These sensors have already proven their ability to collect elevation models with vertical accuracies consistent with 1-, 2- and 4-foot contour intervals in statewide projects. There is no reason to believe the tactics and strategies for statewide collects can’t be expanded to provide complete coverage for the nation in a timely and cost-effective manner.

A nationwide high-accuracy elevation data set that provides a current snapshot of the developed environment would provide many benefits. Several things immediately jump to my mind, like gaining a better understanding of the recent flooding in the Midwest, evaluating the protection afforded by the vast levee systems throughout the country and putting more accurate information in the hands of response teams in times of a national emergency. Similarly, a better understanding of the coastal environment, including the effects of sea level rise over time or the threats associated with the landfall of major storm events, can be gained from accurate elevation data.

This data would also be beneficial in the creation of a nationwide imagery program. Digital orthophotos require an elevation surface during the rectification of the imagery, a task that removes all relief displacement caused by terrain. Many large orthophoto programs today rely on the NED as that elevation surface, but there is a significant addition to the error budget for such imagery products because of the lack of accuracy and currency of the elevation data in the NED. A new elevation surface would be invaluable in creating a new high-accuracy, high-resolution image coverage of the nation.

The thought of new high-accuracy elevation data is exciting. The task before us is critically important. This information, once complete, would be invaluable to the public and private sectors in many different ways.

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