A Fraser Filter is typically used in geophysics when displaying VLF data. It is effectively the first derivative of the data.
If represents the collected data then is the average of two values. Consider this value to be plotted between point 1 and point 2 and do the same with points 3 and 4:
If represents the space between each station along the line then is the Fraser Filter of those four values.
Since is constant, it can be ignored and the Fraser Filter considered to be .