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Digital Elevation Models (DEM's)

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A digital elevation model (DEM) is a digital file consisting of terrain elevations for ground positions at regularly spaced horizontal intervals. The USGS produces five different digital elevation products. Although all are identical in the manner the data are structured, each varies in sampling interval, geographic reference system, areas of coverage, and accuracy; with the primary differing characteristic being the spacing, or sampling interval, of the data. For this project we have employed the7.5-Minute DEM with 30- x 30-meter data spacing. Elevation values for the DEM are derived from vector hypsographic and hydrographic data. The horizontal accuracy of the DEM is approximately one half the contour interval of the source data. Our original topographic maps had a ten-foot contour interval and thus our vertical accuracy is approximately five feet.

DEM's may be used in the generation of three-dimensional graphics displaying terrain slope, aspect (direction of slope), hillshaded images and terrain profiles between selected points. Non-graphic applications such as modeling terrain and gravity data for use in the search for energy resources, calculating the volume of proposed reservoirs, and determining landslide probability have also been developed.

To visualize the potential effects of sea level rise in the NYC region, we have created a hillshaded view of the four 7.5’ quadrangles files that comprise the NYC region. A hillshade view is the hypothetical illumination of a surface. Applying a grayscale color value to the cells in the grid creates the image. The grayscale value is determined by setting an imaginary azimuth and sun angle for the light source. Thus by setting the sun in the south, south-facing sloes would be assigned the brightest values and north-facing slopes will get the darkest values.

The resulting file gives us an image of the topography of the landscape with out any man-made features or vegetation. These models can allow us to see which coastal areas are most susceptible to increases in mean sea-level elevation. The map below shows the result of a query to the DEM to color red all elevations between 0 and three feet. These costal areas are likely to be subjected to the consequences of global warming including sea-level rise and increased exposure to costal storms and flooding. The potential impacts to the area major airports, Kennedy and La Guardia, can easily been seen.

ArcView Spatial Analyst software by Environmental Systems Research Institute (ESRI) was used to create this hillshaded image of the Metropolitan Region.

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