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arc-second (about 10 meters) are available for most areas outside of Alaska, and even



arc-second data (about 3 meters) are available for some areas. These higher resolution data

may become more widely available in the future. The appropriate horizontal resolution for the

input terrain data and receptor network should be determined in consultation with the reviewing

authority based on the specific needs of the project. Higher resolutions for both the terrain data

and receptor network may be necessary in areas with significant terrain relief than for areas with

relatively flat terrain. While acceptable, using the highest resolution elevation data available for

determining receptor elevations and hill height scales may not always be justified. Since spatial

coverage of terrain data for some resolutions may not be complete, it is also worth noting that

use of a single resolution across the domain has advantages, and AERMAP places some

restrictions on the order of DEM or NED file inputs when mixed resolution data are used.

Regardless of the receptor and terrain data resolutions used in AERMAP, it is advisable to check

the accuracy of receptor elevations and hill height scales being input to AERMOD for significant

terrain features that are likely to be associated with peak concentrations based on proximity and

elevation in relation to the sources. Elevations for fenceline or other nearby receptors located

within areas that have been altered due to facility construction may require special consideration

since these changes in local topography may not be reflected in the USGS terrain files. Use of

receptor elevations derived from plant survey data may be an acceptable alternative in these

cases. The option available in AERMAP for the user to provide elevations may be utilized to

determine hill height scales for these special cases, rather than the default option of determining

elevations and hill height scales based on the input terrain data. However, care should be

exercised to ensure that the hill height scales determined by AERMAP are also representative of

the modified topography. If alternative data sources and/or methods are used to estimate

receptor elevations, users must recognize that receptor elevations input to AERMOD should

represent the best estimate of the actual terrain elevation at the receptor location. Use of a

"conservative" estimate of the maximum elevation in the vicinity of the receptor location, such



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