ST_HexagonGrid geom vector to find all points

问题

I was reviewing this function in PostGis

https://postgis.net/docs/manual-dev/ST_HexagonGrid.html

1) What I don't understand is what the underlying geom data would be. What's the source to get the USA map as shown? What is the DB schema? I think it could be one record, if I only need the USA boundary, not each state?

2) Is the result a list of points? or geom vectors?

3) If geom vectors, how do you convert them into points of lat and lng?

4) How to do approximate the hexigons to approximate a 50 mile radius from a point?

回答1:

Can you check this function? I believe it is what you want:

CREATE OR REPLACE FUNCTION generate_hexgrid(width float, xmin float, ymin float, xmax float, ymax float, srid int default 3857)
RETURNS TABLE(gid text,geom geometry(Polygon)) AS $$
DECLARE
  b float := width / 2;
  a float := tan(radians(30)) * b;
  c float := 2 * a;
  height float := 2 * (a + c);
  index_xmin int := floor(xmin / width);
  index_ymin int := floor(ymin / height);
  index_xmax int := ceil(xmax / width);
  index_ymax int := ceil(ymax / height);
  snap_xmin float := index_xmin * width;
  snap_ymin float := index_ymin * height;
  snap_xmax float := index_xmax * width;
  snap_ymax float := index_ymax * height;
  ncol int := abs(index_xmax - index_xmin);
  nrow int := abs(index_ymax - index_ymin);
  polygon_string varchar := 
    'POLYGON((' || 0 || ' ' || 0 || ' , ' || b || ' ' || a || ' , ' ||
    b || ' ' || a + c || ' , ' || 0 || ' ' || a + c + a || ' , ' ||
    -1 * b || ' ' || a + c || ' , ' || -1 * b || ' ' || a || ' , ' ||
    0 || ' ' || 0 ||'))';
BEGIN
  RETURN QUERY
  SELECT 
    format('%s %s %s', width,
    x_offset + (1 * x_series + index_xmin),
    y_offset + (2 * y_series + index_ymin)),
    ST_SetSRID(ST_Translate(two_hex.geom,
    x_series * width + snap_xmin,
    y_series * height + snap_ymin), srid)
  FROM  generate_series(0, ncol, 1) AS x_series,
        generate_series(0, nrow, 1) AS y_series,
    (SELECT 0 AS x_offset, 0 AS y_offset, polygon_string::geometry AS geom
     UNION
     SELECT 0 AS x_offset, 1 AS y_offset, ST_Translate(polygon_string::geometry, b , a + c)  AS geom
    ) AS two_hex;
END; $$ LANGUAGE plpgsql;

According to the author, it should create a grid with the extent based on the given BBOX and the cell size in meters.

SRID 3857 units are [very approximately] meters, and using this projection will create hex cells that "look right" on a web map (most of which use a web mercator projection).

Considering you have a table called usa and it contains the geometries of this shapefile you should be able to create a grid that overlaps the USA map with the following query:

CREATE TABLE usa_hex AS
WITH j AS (
SELECT ST_Transform((hex).geom,4326) AS hex FROM ( 
  SELECT 
  generate_hexgrid(
    80467,
    ST_XMin(ST_Extent(ST_Transform(geom,3857))) ,
    ST_YMin(ST_Extent(ST_Transform(geom,3857))) ,
    ST_XMax(ST_Extent(ST_Transform(geom,3857))) ,
    ST_YMax(ST_Extent(ST_Transform(geom,3857))) ) AS hex
FROM usa)i) 
SELECT j.hex FROM j,usa
WHERE ST_Intersects(usa.geom,j.hex);

来源：https://stackoverflow.com/questions/59848162/st-hexagongrid-geom-vector-to-find-all-points

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