agencies to acquire high-resolution, high-accuracy topographic data throughout the nation.

  • National Water Information System. Stream gage data, available through the USGS National Water Information System, provide the necessary riverine discharge information required for flood studies. Flood maps can be produced with much greater accuracy when a long and consistent history of stream gage information, and therefore discharges during flooding, is available.

  • USGS Storm Surge Network. The USGS currently deploys short-duration storm surge gages prior to expected landfall of hurricanes. These gages are a considerable improvement over post-storm watermark surveys, which are subject to significant errors and uncertainties in the peak storm surge and wave conditions. Accurate storm surge measurements are critical for verifying coastal storm surge models using selected historical storms (see Chapter 5).

  • National Water Level Observation Network. Flood risk is increasing rapidly in coastal areas due to a combination of land subsidence, sea level rise, population growth, and development. Coastal water elevations, measured and monitored through NOAA’s NWLON program, provide essential information for FEMA’s coastal flood maps. The information provided by NWLON tide gages is also critical to the development of VDatum, which in turn is needed to develop seamless topographic-bathymetric surfaces for coastal flood studies.

Elevation and height data are analogous to the foundation of a skyscraper; even if the engineering design and construction are flawless, the entire building is at risk of failure if the foundation is inadequate. It would be wise to lay a strong foundation before investing additional time, effort, and money in further construction of a building. Yet we have not taken such an approach to elevation data as they pertain to floodplain mapping. The technology and knowledge to build and maintain a comprehensive and accurate elevation measurement system have been available for 15 to 20 years. The main hurdle to implementing such a system nationwide has been cost. The relative costs and benefits of investing substantially in elevation data to produce more accurate flood maps are discussed in Chapter 6.

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