Topographic Change SRTM NED Vertical Differencing

Diagram of the SRTM - NED coregistration process. Prior to registration, each NED pixel (red, on left side diagram) is centered on four SRTM pixels (shaded, on left side diagram). These four SRTM pixels are averaged and the resulting value is placed into the upper left position of the 2x2-degree window. After the averaging is completed for the entire SRTM grid, the output is shifted one-half pixel in both the x and y directions to align the SRTM pixels with the NED pixels (right side diagram).