Graham McIntire
5a9359191f
Eliminates the external wgrib2 C tool dependency that blocked HRRR processing. Implements Lambert Conformal projection, simple packing (Template 5.0), complex packing with spatial differencing (Template 5.3), and GRIB2 section parsing — enough to extract point values from HRRR grid data using only Elixir.
84 lines
2.1 KiB
Elixir
84 lines
2.1 KiB
Elixir
defmodule Microwaveprop.Weather.Grib2.LambertConformal do
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@moduledoc false
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@r_earth 6_371_229.0
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@doc """
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Convert a lat/lon to grid indices (i, j) on a Lambert Conformal Conic grid.
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Returns `{:ok, {i, j}}` or `{:error, :outside_grid}`.
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"""
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def to_grid_index(grid, lat, lon) do
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%{
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nx: nx,
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ny: ny,
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la1: la1,
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lo1: lo1,
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dx: dx,
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dy: dy,
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latin1: latin1,
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lov: lov
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} = grid
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# Normalize longitudes to 0..360
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lo1 = normalize_lon(lo1)
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lon = normalize_lon(lon)
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lov = normalize_lon(lov)
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latin1_rad = deg_to_rad(latin1)
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la1_rad = deg_to_rad(la1)
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lat_rad = deg_to_rad(lat)
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# Cone constant (for HRRR, latin1 == latin2 == 38.5, so n = sin(latin1))
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n = :math.sin(latin1_rad)
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# F factor
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f =
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:math.cos(latin1_rad) *
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:math.pow(:math.tan(:math.pi() / 4 + latin1_rad / 2), n) / n
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# rho for a given latitude
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rho_origin = rho(la1_rad, n, f)
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rho_target = rho(lat_rad, n, f)
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# Theta angles (difference from central meridian, scaled by cone constant)
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theta_origin = n * deg_to_rad(lo1 - lov)
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theta_target = n * deg_to_rad(adjusted_lon_diff(lon, lov))
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# Grid coordinates
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x_origin = rho_origin * :math.sin(theta_origin)
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y_origin = rho_origin * :math.cos(theta_origin)
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x_target = rho_target * :math.sin(theta_target)
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y_target = rho_target * :math.cos(theta_target)
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i = round((x_target - x_origin) / dx)
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j = round((y_origin - y_target) / dy)
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if i >= 0 and i < nx and j >= 0 and j < ny do
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{:ok, {i, j}}
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else
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{:error, :outside_grid}
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end
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end
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defp rho(lat_rad, n, f) do
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@r_earth * f / :math.pow(:math.tan(:math.pi() / 4 + lat_rad / 2), n)
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end
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defp deg_to_rad(deg), do: deg * :math.pi() / 180.0
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defp normalize_lon(lon) when lon < 0, do: lon + 360.0
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defp normalize_lon(lon), do: lon + 0.0
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# Compute longitude difference handling wrap-around
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defp adjusted_lon_diff(lon, lov) do
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diff = lon - lov
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cond do
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diff > 180.0 -> diff - 360.0
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diff < -180.0 -> diff + 360.0
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true -> diff
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end
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end
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end
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