This document describes the process for visualizing data on spherical grids.
A spherical grid data set is indicated by the VDF CoordinateSystem keyword having a value of "spherical". This keyword can be specified when creating a .vdf file via the vfdcreate "-coordsystem" option.
The 3-element Metadata DimensionLength attribute specifies the dimension, in grid points, of the spherical data set. Dimensions are always specified in order of the fastest to slowest varying coordinate axes. By default the fastest varying dimension is longitude, followed by latitude, and then radius. Thus, for example, a dimension of 256x128x64 indicates a grid with 256 points in longitude, 128 points in latitude, and 64 points in the radial dimension. Furthermore, longitude increases from West to East, latitude from North to South, and radius from inner to outer.
If the data are not organized with longitude varying fastest, (followed by latitude then radius) the VDF GridPermutation keyword may be used to indicate a non-standard order: a permutation of the default order. This keyword can be specified when creating a .vdf file via the vdfcreate -order option. The argument to -order specifies a three- element, integer vector whose contents define a permutation of the data order. Longitude is mapped to 0, latitude to 1, and radius to 2. Thus the default permutation is the vector: 0 1 2. If data are organized with radius varying fastest and longitude slowest, for example, the permutation vector 2 1 0 should be used.
Physical extents of data are specified with the six-element VDF Extents keyword. This keyword can be specified when creating a .vdf file via the vdfcreate -extents keyword. Angular measures are specified in degrees, and radius is unit-less. The first three elements of the vector specify the minimum data extent, the last three specify the maximum data extent. Ordering within the three-element min and max subvectors is determined by the GridPermutation keyword. For example, under the default permutation {0, 1, 2} the min and max longitude are given by v [0] and v[4], respectively. If the permutation is {2,1,0} the values v [0], and v[4] would give the min and max radial extents, respectively. The default extents are:
minimum longitude = -180 + (360 / NX/2)
maximum longitude = 180 - (360 / NX/2)
minimum latitude = -90 + (180 / NY/2)
maximum latitude = 90 - (180 / NY/2)
minimum radius = 0.0
maximum radius = 0.5
Where NX and NY are the dimension, in grid points, of the longitude and latitude coordinates, respectively.
Finally, the 3-element VDF PeriodicBoundary keyword determines the periodicity of the data boundaries. This keyword can be specified when creating a .vdf file via the vdfcreate -periodic keyword. The ordering of the elements for this keyword is subject to the GridPermutation keyword. Under the default permutation (0,1,2) the periodicity is (1,1,0). That is, the longitude and latitude boundaries are considered periodic. Specifying a periodic radial boundary has no meaning and is presently ignored.
Given data on a spherical shell with radius from 0.75 to 1.0 and dimensions 64x180x360 (radius x latitude x longitude) the vdfcreate command would be:
vdfcreate -dimensio 64x180x360 -order 2:1:0 -periodic 0:1:1 -extents 0.75:-89.5:-179.5:1.0:89.5:179.5 ...
Given the Eastern hemisphere of a solid sphere with dimensions 180x180x64 (longitude x latitude x radius) the vdfcreate command would be:
vdfcreate -dimensio 180x180x64 -order 0:1:2 -periodic 0:1:0 -extents 0.0:-89.5:0:1.0:89.5:180.0 ...
