Question

This is my database table (spots).

Given a point on map P(longitude, latitude), what I want to retrieve from the database is the set of points within a given radius where P is the center. I tried a simple query using the simple formula of Longitude and Latitude, unfortunately the result is always null 🙁 . What am I doing wrong here? Any idea?

$sql = "SELECT * FROM $tbl_name 
WHERE ACOS(SIN('".$latitude."') * SIN(spots.latitude) + COS('".$latitude."') * COS(spots.latitude) * COS(spots.longitude - '".$longitude    ."')) * 6371 <= '".$radius."'";

Answer 1

Based on Cameron S’s & MrGlass’s ideas, and a bit searching over the Internet, I found one working solution:

$R = 6371;
$max_lat = $lat + rad2deg($r/$R);
$min_lat = $lat - rad2deg($r/$R);

$max_lon = $lon + rad2deg($r/$R/cos(deg2rad($lat)));
$min_lon = $lon - rad2deg($r/$R/cos(deg2rad($lat)));
$lat = deg2rad($lat);
$lon = deg2rad($lon);

$sql = "
select id, name, description, latitude, longitude, 
       acos(sin($lat)*sin(radians(latitude)) + cos($lat)*cos(radians(latitude))*cos(radians(longitude)-$lon)) * $R as D
from (
  select id, name, description, latitude, longitude 
  from $tbl_name 
  where latitude > $min_lat and latitude < $max_lat
    and longitude > $min_lon and longitude < $max_lon
  ) as first_cut 
where acos(sin($lat)*sin(radians(latitude)) + cos($lat)*cos(radians(latitude))*cos(radians(longitude) - $lon)) * $R < $r
order by D";