52 lines
1.9 KiB
Python
52 lines
1.9 KiB
Python
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"""Snap an arbitrary lat/lon to a stable ~4-square-mile grid cell.
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The grid is defined by fixed latitude rows (~2 miles tall). Within each row the
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longitude step is scaled by cos(latitude) so cells stay roughly square (~4 sq mi)
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at every latitude instead of getting skinny toward the poles. Cell ids are
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deterministic, so the same physical location always maps to the same cache file.
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"""
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import math
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# 1 degree of latitude ~= 69 miles. ~2 miles -> ~0.029 deg gives a ~4 sq mi cell.
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LAT_STEP = 1.0 / 34.5 # ~= 0.02899 deg (~2.0 miles)
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def _lon_step(center_lat: float) -> float:
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"""Longitude degrees that span ~2 miles at the given latitude."""
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c = math.cos(math.radians(center_lat))
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c = max(c, 0.05) # clamp near the poles to avoid a blow-up
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return LAT_STEP / c
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def snap(lat: float, lon: float) -> dict:
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"""Return the grid cell (id + center + span) containing (lat, lon)."""
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i = math.floor(lat / LAT_STEP)
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center_lat = (i + 0.5) * LAT_STEP
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lon_step = _lon_step(center_lat)
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j = math.floor(lon / lon_step)
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center_lon = (j + 0.5) * lon_step
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# Approximate cell dimensions in miles for display.
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height_mi = LAT_STEP * 69.0
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width_mi = lon_step * 69.0 * math.cos(math.radians(center_lat))
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return {
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"id": f"{i}_{j}",
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"center_lat": round(center_lat, 5),
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"center_lon": round(center_lon, 5),
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"lat_step": LAT_STEP,
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"lon_step": lon_step,
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"bounds": {
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"south": round(i * LAT_STEP, 5),
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"north": round((i + 1) * LAT_STEP, 5),
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"west": round(j * lon_step, 5),
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"east": round((j + 1) * lon_step, 5),
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},
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"area_sq_mi": round(height_mi * width_mi, 2),
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}
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def in_north_america(lat: float, lon: float) -> bool:
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"""Rough bounding box for the US (incl. Alaska/Hawaii) and Canada."""
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return 14.0 <= lat <= 84.0 and -172.0 <= lon <= -52.0
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