Leaflet draw shapes and get the overlapped/included zipcodes

Question

I am new to the GIS world but I have experience in programming. I am trying to build an app where users draw circles on a leaflet map and it returns the zipcodes "touched", but I think the right term is overlapped, by the circles. I have a table in PostGIS where I have uploaded the US census zipcodes; the column to query is a geometry type which return something like this when transformed as GeoJSON:

{"type":"MultiPolygon","crs":{"type":"name","properties":{"name":"EPSG:4269"}},"coordinates":[[[[-64.903808,17.679739],[-64.903804,17.679763],[-64.903799,17.679801],[-64.903796,17.67988],[-64.903793,17.679919],[-64.903788,17.679997],[-64.90377,17.680077],[-64.903756,17.680154],[-64.903744,17.680199],[-64.903729,17.68024],[-64.9037,17.68031]...}

This is the table:

And this is an example of the map with a circle drawn where I console log its radius and center coordinates (lat, lng):

So far I have tried this query:

SELECT * 
FROM tl_2019_us_zcta510 AS b
WHERE ST_DWithin(b.geom,
  ST_Transform(
    ST_SetSrid(
      ST_MakePoint(lat_from_leaflet,lng_from_leaflet)
      ,4269)
    ,4269)
,radius_from_leaflet);

This isn't returning the correct zipcodes because when I try with a small circle, for example, it doesn't return any row, but I am expecting at least one row.

Answer 1

Two issues:

• almost everywhere in OGC supported environments, coordinate order is {X, Y} | {lon, lat}
• your radius seems to be given in meter, while ST_DWithin (as almost all functions in PostGIS) assume the primary unit of the CRS of the given geometry - which is degree for EPSG:4269! In order to calculate proximity with meter as reference unit you'd need to either use a suitable projection, or the GEOGRAPHY type with a defined geographic CRS; I assume here that the input values are referenced in EPSG:4326:

  SELECT
    *
  FROM
    <table> AS t
  WHERE
    ST_DWithin(
      t.geom::GEOGRAPHY(GEOMETRY, 4269),
      ST_Transfrom(ST_SetSRID(ST_MakePoint(<lon>, <lat>), 4326), 4269)::GEOGRAPHY(GEOMETRY, 4269),
      <radius_in_meter>
    )
  ;
  

  You'd want to have this covered by an appropriate index:

  CREATE INDEX ON <table> USING GIST (( geom::GEOGRAPHY(GEOMETRY, 4269) ));