It is clearly stated that the method NGA used to produce 3 arcsecond
data from the 1 arc-second set is "subsampling", namely
selecting the center value from the set of nine centered on a
particular posting location (SRTM FAQ, 2004) as shown in
figure 2 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.