Latitude-longitude grid

Dictionary: grid_params · Class: GridParams

Used by: planet, trace

Defines the latitude/longitude grid over which a global field/cutoff map is computed.

Key	Type	Default	Description
maxlat	float	90	Northern latitude bound (degrees).
minlat	float	-90	Southern latitude bound (degrees).
latstep	float	-5	Latitude step (degrees). Must be negative to iterate from maxlat down to minlat — a positive value triggers a warning and will likely produce an empty or unexpected grid.
maxlong	float	360	Eastern longitude bound (degrees).
minlong	float	0	Western longitude bound (degrees).
longstep	float	5	Longitude step (degrees), from minlong up to maxlong.
array_of_lats_and_longs	list or None	None	Optional explicit list of (latitude, longitude) pairs to use instead of generating a grid from the bounds/step above.
grid_type	str	"regular"	planet only. "regular" builds the latitude/longitude grid above. "healpix" uses equal-area HEALPix pixel centres. "healpix_geomag" uses the same points with the rings laid out in centred-dipole (geomagnetic) coordinates for the requested date. "fibonacci" uses a golden-angle (Fibonacci) spiral. The equal-area types ignore the steps and bounds.
nside	int	12	Resolution for the equal-area grid types: 12 * nside**2 points (the HEALPix count, also used for "fibonacci").

Equal-area grids

A regular grid puts the same number of points in every latitude row, so the points crowd together towards the poles. A 5° grid has 29% of its points poleward of ±60°, which is only 13% of the Earth's surface, and the cutoff there is near zero and barely changes. HEALPix points each cover the same area.

nside	Points	Spacing
10	1200	~650 km
12	1728	~550 km (like 5° at the equator)
16	3072	~410 km
23	6348	~290 km

The points are built as equal-area directions from the Earth's centre (geocentric). With inputcoord "GDZ" (the default) their latitudes are converted to WGS84 geodetic latitude, which moves them up to ~0.19° poleward at mid-latitudes. With "GEO" or "SPH" they stay geocentric.

The points are not on a latitude/longitude grid. To map them, interpolate on the sphere. A smooth interpolant, e.g. scipy.interpolate.RBFInterpolator with a cubic kernel on the 3-D unit vectors, worked best for Ru and Rc, and linear interpolation worked best for Rl and the transmission.

import OTSO

df, *_ = OTSO.planet(grid_params={"grid_type": "healpix_geomag", "nside": 12})