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RECONSTRUCTION OF 30 M DEM FROM 90 M SRTM DEM WITH BICUBIC POLYNOMIAL INTERPOLATION METHOD

This work presents a methodology for the refinement of 90m digital elevation model (DEM) of Shuttle Radar Topographic Mission (SRTM) data. The SRTM DEM is publicly available through the Seamless Data Distribution System (SDDS) at two resolutions: 30m for the United States, and 90m for the areas between 60N and 56S of latitude including the US. The selected test areas stretch eastwest from gently-slope flat land in western part of Kansas (N40W102.5) to rugged mountainous region in eastern Colorado (N40W107). All 90m DEM files, however, are generated from the 30m DEMs by subsampling only the center value of each 3x3 cells of the 30m SRTM DEM. Thus, the 90m DEM can be regarded as, in interpolation sense, regular sampling points that can be used to generate height values over a finer grid, which is set to be 30m in this study. Each height value is computed by choosing 16 nearest sampling points to determine 10 parameters of bivariate cubic polynomial equations in a least-square fashion. The reconstructed 30m DEMs are compared with the 30m SRTM DEMs. The interpolation errors, defined as the height differences at the same location between the 30m SRTM DEMs and the corresponding interpolating DEMs, are computed. Statistics show that the interpolation results are extremely good. For all reconstructed 30m DEM files, the RMS errors of the DEM files N40W102.5 N40W107 are 0.936, 1.024, 1.081, 0.908, 0.817, 1.207, 1.923, 2.307, 2.330 and 1.865m respectively. The files having larger RMS error are those covering mountainous areas in the west (N40W105.5 N40W107) while those in the flat and smooth terrains have RMS error better than 1.2 m. The result clearly demonstrates that for most cases using 90m SRTM DEMs as a source, the generation of 30m resolution DEMs of the same quality as the original 30m SRTM DEMs can be achieved by using the bicubic polynomial interpolation technique. This would be beneficial for a great number of users whose working areas are outside the US and requires higher resolution DEMs than the 90m SRTM DEM currently available.