Knowing Your Elevation

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".

• Hire a surveyor to get an elevation accurate to an inch. Cost: $200 - $500.
• High-accuracy lidar data is available at the NOAA Coastal Services Center. I picked a section for New Smyrna Beach, Florida, then ordered a 7Mb file in .tiff format that I was able to view using Photoshop. I had to wait about 1/2 hour for the file to be prepared. Lidar eleveation data for the entire coast of Florida should be available by 2010.
• University of Arizona Climate Change and Sea Level interactive tool. The data is taken from the USGS National Elevation Dataset. Accuracy: 3 - 30 meters in the conterminus U.S., and 30 meters for the rest of the globe. The tool has a nice feature where one can choose how sea level rises of 1, 2, 3, 4, 5, or 6 meters would affect the coast.
• Sea Level Rise Explorer by Global Warming Art. Global interactive Google Map, which lets you zoom down to the street level. Accuracy: 5 - 9 meters. Coverage: entire globe.
• United States Geological Survey (USGS) National Elevation Dataset. Accuracy: 3 - 30 meters. A zoomable interactive map lets you click on your location and see the interpolated elevation. Don't be fooled, though, when you click and see an elevation of 5.28 feet in a region where the resolution of the data is only 10 meters (1/3 arc second, 33 feet). The real elevation could easily be 1 foot, or 30 feet.
• EPA sea level rise pages for the U.S. Atlantic coast. Shows the modeled areas of elevations below 1.5 meters, and 1.5 - 2.5 meters. No roads are overlaid, so it's a bit hard to tell what cities are impacted.

Lesser Accuracy

• Google Earth: The best elevation accuracy of Google Earth in the U.S. is 10 meters.
• GPS: Global Positioning System (GPS) is not a good choice, since the vertical accuracy of most hand-held GPS units for home use is only 15 meters (49 feet). If you have access to a commercial grade differential GPS system (a minimum of $5000), one can get an accuracy on the order of a few centimeters.