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Knowing Your Elevation
By Jeffrey Masters, Ph.D. — Director of Meteorology, Weather Underground, Inc.
If you live near the coast, or are thinking of buying property near the coast, it's very much in your financial and survival interests to know exactly what elevation your home is at. If you are a home owner, your flood insurance certificate should tell you what your elevation is, based on the best available USGS surveys for the area. In some cases, though, this information is not very precise. For example, in the New York City area, elevations as of 2008 on USGS topographic maps were only surveyed to an accuracy of 10 feet (Figure 1). Along the Hudson River and some portions of Long Island, the accuracy was only 20 feet. These topographic maps have a considerable error range, too, with 90% of the data rated as accurate to plus or minus half the contour interval. Thus, this means that 90% of the points along a 10-foot contour line lie in the range 5 - 15 feet. This is a pretty broad range if you're trying to judge your vulnerability to a storm surge. Efforts are being made in many areas to perform high-resolution mapping using laser measurements from aircraft combined with GPS. These data sets generally have an accuracy of 0.15 meters (six inches), but the data is limited in coverage and difficult to find on the Internet. Probably the best solution is to use existing low-resultion data (3 - 30 meters in the U.S.) and interpolate the data to your location. Keep in mind that the errors will often be large using these techniques.
Figure 1. The current best available elevation source data (as of August 2008) for the USGS National Elevation Dataset over the mid-Atlantic region. Lidar data typically has an accuracy of 0.15 meters. Image credit: Coastal Sensitivity to Sea-Level Rise: A Focus on the Mid-Atlantic Region report by the U.S. Climate Science Program.
Sources of Elevation Information
The flood insurance certificate for your property, and even some mortgage documents, should have a surveyed elevation reading. According to Dr. Stephen Baig, retired head of NHC's storm surge team, "these are conventionally measured by a professional surveyor using government-maintained elevation survey bench marks. hose benchmarks are very precise and accurate references provided the ground is stable and the mark is well anchored. In some areas, the benchmark data are of questionable quality. In large, the quality of coastal elevation marks is not good. That's because they lie on the edges of a network of gauges. Within the network there is a more-or-less self-correcting phenomenon going on. At the edges the gauges are out on "spurs", tied to the net by only two other shoreward gauges. No gauges offshore, either, to help "close" the network. So elevation data right at the shoreline, exactly where it matters a great deal to surge calculations, are of questionable quality. The shoreline of the Gulf of Mexico, extending from the Florida border westward to Brownsville (and into Mexico, too) presents an additional elevation problem. It's subsiding, sliding down into the Gulf of Mexico, headed for the Yucatan. The vertical reference network for this region is destroyed. This is absolute change, on top of which there is sea-level rise. The best that can be done is the real-time kinetic elevation data program established and managed by Louisiana Spatial Reference Center (LSRC) Center for Geoinformatics ( http://c4g.lsu.edu/joomla/)". Without their help we would not have the confidence we do in the SLOSH calculations for that region".
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