"CMP stacking" is a robust process that uses the fact that a particular location in the subsurface will have been sampled numerous times and at different offsets. This allows a geophysicist to construct a group of traces with a range of offsets that all sample the same subsurface location, known as a "Common Midpoint Gather". The average amplitude is then calculated along a time sample, resulting in significantly lowering the random noise but also losing all valuable information about the relationship between seismic amplitude and offset. Less significant processes that are applied shortly before the "CMP stack" are "Normal moveout correction" and "statics correction". Unlike marine seismic data, land seismic data has to be corrected for the elevation differences between the shot and receiver locations. This correction is in the form of a vertical time shift to a flat datum and is known as a "statics correction", but will need further correcting later in the processing sequence because the velocity of the near-surface is not accurately known. This further correction is known as a "residual statics correction."