/// <summary>
/// Radius of bounding box in hexagons. i.e., the radius of a k-ring centered
/// on the bbox center and covering the entire bbox.
/// </summary>
/// <param name="bbox">Bounding box to measure</param>
/// <param name="res">Resolution of hexagons to use in measurement</param>
/// <returns>Radius in hexagons</returns>
/// <!-- Based off 3.1.1 -->
public static int bboxHexRadius(BBox bbox, int res)
{
// Determine the center of the bounding box
GeoCoord center = new GeoCoord();
bboxCenter(bbox, ref center);
// Use a vertex on the side closest to the equator, to ensure the longest
// radius in cases with significant distortion. East/west is arbitrary.
double lat =
Math.Abs(bbox.north) > Math.Abs(bbox.south)
? bbox.south
: bbox.north;
GeoCoord vertex = new GeoCoord {
lat = lat, lon = bbox.east
};
// Determine the length of the bounding box "radius" to then use
// as a circle on the earth that the k-rings must be greater than
double bboxRadiusKm = GeoCoord._geoDistKm(center, new [] { vertex });
// Determine the radius of the center hexagon
double centerHexRadiusKm = _hexRadiusKm(H3Index.geoToH3(ref center, res));
// The closest point along a hexagon drawn through the center points
// of a k-ring aggregation is exactly 1.5 radii of the hexagon. For
// any orientation of the GeoJSON encased in a circle defined by the
// bounding box radius and center, it is guaranteed to fit in this k-ring
// Rounded *up* to guarantee containment
return((int)Math.Ceiling(bboxRadiusKm / (1.5 * centerHexRadiusKm)));
}
/// <summary>
/// Returns the radius of a given hexagon in kilometers
/// </summary>
/// <param name="h3Index">Index of the hexagon</param>
/// <returns>radius of hexagon in kilometers</returns>
/// <!-- Based off 3.1.1 -->
static double _hexRadiusKm(H3Index h3Index)
{
// There is probably a cheaper way to determine the radius of a
// hexagon, but this way is conceptually simple
GeoCoord h3Center = new GeoCoord();
GeoBoundary h3Boundary = new GeoBoundary();
H3Index.h3ToGeo(h3Index, ref h3Center);
H3Index.h3ToGeoBoundary(h3Index, ref h3Boundary);
return(GeoCoord._geoDistKm(h3Center, h3Boundary.verts));
}