The first step in the process of assembling NED is the
identification and acquisition of all DEM files available for the geographic
area being assembled.
This step was designed to be automated and to work for a
variable geographic extent. The process was implemented by developing a
query tool that scanned selected DEM availability indices and returned the
name and location of the files found within the specified region.
The query tool that was developed required the compilation
of DEM availability indices for all existing data sources
(7.5-minute,30-minute,1-degree,SAST).
The availability indices were derived from a database
search of the NMD Sales Database (SDB). The latest search provided a list of
the 50,000 DEM files currently in the SDB. The list is used to extract and
store the header record from each DEM. The resulting list of DEM header
records is subsequently processed to create DEM availability indices, GIS
polygon files, and associated attribute tables.
The information extracted from the DEM header record
database can be easily redefined and currently includes the full DEM file
and path name, state code, production method, horizontal datum, vertical
datum, elevation units, SW corner coordinates, DEM level, production codes,
and mapping center codes.
Understanding the varying data characteristics of existing DEM's was
necessary to determine the data conversions and transformations required to
assemble the NED database. National digital elevation data production began
over 20 years ago.
The data produced have varied in production method and
data quality. Varying specifications have lead to the production of DEM's
with different horizontal datums, vertical datums, and elevation units. Many
of these inconsistencies were eliminated during NED processing.
(courtesy of the USGS)
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