User’s Guide Addendum (EPA, 2009) provides a more detailed discussion of issues associated

with gaps between DEM files or within DEM files, and describes how these cases are handled by


While NED is considered an improvement in the quality and consistency of elevation data for

use with AERMAP, there are some issues associated with the GeoTIFF format supported by

AERMAP that users should be aware of. The main issue of importance to AERMAP users is

that the NED GeoTIFF files currently available from the USGS Seamless Data Server do not

include the GeoKey specifying the units for the elevation data. The USGS documentation for

NED data (USGS, 2002) indicates that elevations are in units of meters and are provided in

floating point format. AERMAP will therefore assume units of meters if the elevation units

GeoKey is absent. However, non-standard (i.e., non-USGS) NED data in GeoTIFF format may


be in units of meters. AERMAP provides an option for users to specify elevation units in

these cases. However, users must exercise caution in using such data unless the correct units can

be confirmed. The AERMAP User’s Guide Addendum (EPA, 2009) provides a more detailed

discussion of these and other potential issues associated with the GeoTIFF format supported for

NED data.

The NED elevation data are currently available for the conterminous United States, Hawaii,

Puerto Rico, and the Virgin Islands at a horizontal resolution of 1 arc-second (approximately 30

meters), and at a resolution of 2 arc-seconds for Alaska. Higher resolution NED elevation data


n201 - n202 - n203 - n204 - n205 - n206 - n207 - n208 - n209 - n210 - n211 - n212 - n213 - n214 - n215 - n216 - n217 - n218 - n219 - n220 - n221 - n222 - n223 - n224 - n225 - n226 - n227 - n228 - n229 - n230 - n231 - n232 - n233 - n234 - n235 - n236 - n237 - n238 - n239 - n240 - n241 - n242 - n243 - n244 - n245 - n246 - n247 - n248 - n249 - n250


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