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) | 
