The customary production of DEM's based on quadrangle boundaries necessitates
the paneling of multiple DEM files to compile data for any large study area.
In building NED, the various source DEM files were appended (or paneled)
together. A 1- x 1-degree tile with 0.125 degrees of overedge required as
many as 100 7.5-minute DEM files, 36 30-minute DEM files, or 9 1-degree DEM
files.
Individual DEM files were read in the standard USGS ASCII format, converted
to binary, rotated so that north was to the top, and placed in the proper
geographic position within a temporary array in memory.
Small slivers of missing data were occasionally found within the seams of
the paneled 7.5-minute image data. A bilinear interpolation algorithm was
used to correct these discrepancies during the assemble process.
The "best available" elevation data was assembled by merging
multiple overlapping data sources. The "best available" data can
be determined by either a logical operation or a priority in the overlay
sequence. Logical operations can include cell based or moving window based
operations of the average, minimum, or maximum elevations found in the
source materials. A priority specification uses a preferred sequence for
merging the elevation values from the input source materials. A priority
sequence based on the highest spatial resolution of the source material is
currently used to define the "best available" data. The 7.5-minute
quadrangles were given highest priority for the output. If 7.5-minute data
didn't exist the 2-arc or 3-arc second data was then used to populate the
output.
(courtesy of the USGS) |
