The USGS DEM archives are now static and will not be updated in the future, while the NED

data are being actively supported and checked for quality. Therefore, NED represents a more

up-to-date and improved resource for terrain elevations for use with AERMAP. Due to a

number of problems that have been encountered with DEM data, AERMOD users are

encouraged to transition to the use of NED data as soon as practicable. Problems encountered

with DEM data include incorrect geo-referencing information for entire DEM files and

elevations that reflect the tops of buildings and trees in some cases. The use of NED data should

avoid these issues, and provides additional advantages over the use of DEM data, including the

ability to download a single NED file to cover the entire modeling domain of interest, with a

consistent horizontal resolution and reference datum (generally NAD83). Some applications of

AERMAP using DEM data may involve inconsistent reference datums for adjacent DEM files,

which can result in receptors being located within gaps between files due to the datum shift.

Gaps may also occur within DEM files generated by various software tools to convert from one

format to another when a NAD conversion is involved, e.g., converting1-degree DEM data to the

7.5-minute DEM format to fill areas not covered by available 7.5-minute data. The AERMAP

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



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