Use ground distance for sky_view_factor horizon angle (#1407)#1410
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The CPU and GPU kernels computed the horizon angle as atan2(dz, r) where r was the integer ray-step index, treating cell size as 1. For any DEM whose cell size differs from the elevation unit (e.g. a 30 m DEM in meters), the angle was wrong by a factor of cell_size, collapsing SVF toward 0 on coarse grids and inflating it on fine grids. The kernels now take cellsize_x and cellsize_y and use the true ground distance sqrt((dx*csx)^2 + (dy*csy)^2). The public API reads cell size from agg.attrs['res'] or the coordinate spacing via get_dataarray_resolution, with optional cellsize_x / cellsize_y overrides. Tests cover non-unit cell size, anisotropic cell sizes, explicit overrides, and confirm flat terrain still gives SVF=1 across cell sizes.
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Summary
atan2(dz, r)withrin cells, which silently assumes cell size 1. For any DEM whose cell size differs from the elevation unit (a 30 m DEM in meters, a sub-meter aerial DEM, etc.) the angle was wrong by a factor of cell size.cellsize_xandcellsize_yinto both kernels and compute the true ground distance per ray step:sqrt((sx - x) * csx)^2 + ((sy - y) * csy)^2).agg.attrs['res']or coordinate spacing viaget_dataarray_resolution. Add optionalcellsize_x/cellsize_yoverrides on the public API.Fixes #1407.
Test plan
test_cellsize_changes_horizon_angleconfirms halving the cell size lowers SVF on a uniform ramptest_cellsize_explicit_overrideconfirms explicit kwargs overrideattrs['res']test_anisotropic_cellsizecoverscellsize_x != cellsize_ytest_flat_surface_unaffected_by_cellsizeconfirms flat terrain still gives SVF=1 across cell sizes