A gaussian grid is a latitude/longitude grid. The spacing of the= latitudes is not regular (see definitions below). However, the spacing of = the lines of latitude is symmetrical about the Equator. Note that there is = no latitude at either Pole or at the Equator. A grid is usually referred to= by its 'number' N, which is the number of lines of latitude between a Pole= and the Equator.

The longitudes of the grid points are defined by giving the number of po= ints along each line of latitude. The first point is at longitude 0 and the= points are equally spaced along the line of latitude. In a regular gaussia= n grid, the number of longitude points along a latitude is 4*N. In a reduce= d gaussian grid, the number of longitude points along a latitude is specifi= ed. Latitudes may have differing numbers of points but the grid is symmetri= cal about the Equator. A reduced gaussian grid may also be called a quasi-r= egular gaussian grid.

In the reduced grids used by ECMWF, the number of points on each latitud= e row is chosen so that the local east-west grid length remains approximate= ly constant for all latitudes, with the restriction that the number should = be suitable for the Fast Fourier Transform used to interpolate spectral fie= lds to grid point fields, ie number =3D 2^p * 3^q * 5^r.

It is possible to supply a gaussian grid definition. The latitude values= defining the grid can be changed or the number of points along each latitu= de can be specfified, or both, as long as the resulting grid definition is = self-consistent. The gaussian latitudes may be given as an array of values = using the option 'g_lats', and the number of points along each line of lati= tude may be given as an array of values using the option 'g_pts'. See INTIN or IN= TOUT.