### Triangular grid based on an icosahedron (see Attachment, Volume I.2, Part B, Att GRIB)

Created 08/03/2005

Octet No. Contents
15
n2 ― exponent of 2 for the number of intervals on main triangle sides
16
n3 ― exponent of 3 for the number of intervals on main triangle sides
17-18
ni ― number of intervals on main triangle sides of the icosahedron
19
nd ― Number of diamonds
20-23
Latitude of the pole point of the icosahedron on the sphere
24-27
Longitude of the pole point of the icosahedron on the sphere
28-31
Longitude of the centre line of the first diamond of the icosahedron on the sphere
32
Grid point position (see Code Table 3.8)
33
Numbering order of diamonds (flags ― see Flag Table 3.9)
34
Scanning mode for one diamond (flags ― see Flag Table 3.1000)
35-38
nt ― total number of great points

 Notes: 1. For more details see Attachment, Volume I.2, Part B, Att. GRIB ― Definition of a triangular grid based on an icosahedron. 2. The origin of the grid is an icosahedron with 20 triangles and 12 vertices. The triangles are combined to nd quadrangle, the so-called diamonds (e.g. if nd = 10, two of the icosahedron triangles form a diamond, and if nd = 5, 4 icosahedron triangles form a diamond). There are two resolution values called n2 and n3 describing the division of each triangle side. Each triangle side is divided into ni equal parts, where ni = 3n3 x 2n2 with n3 either equal to 0 or to 1 . In the example of the Attachment, the numbering order of the rectangles is anti-clockwise with a view from the pole point on both hemispheres. Diamonds 1 to 5 are northern hemisphere and diamonds 6 to 10 are southern hemisphere. 3. The exponent of 3 for the number of divisions of triangle sides is used only with a value of either 0 or 1. 4. The total number of grid points for one global field depends on the grid point position. If e.g. the grid points are located at the vertices of the triangles, then nt = (ni + 1) x (ni + 1) x nd since grid point at diamond edges are contained in both adjacent diamonds and for the same reason the pole mare contained in each of the five adjacent diamonds.

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