St_buffer() giving inaccurate results

Question

I have the following code:

set @lon = 121.4732134;
set @lat =  31.2304321;
set @point = point(@lon, @lat);
set @radius = .5;
set @polygon = ST_Buffer(@point, @radius);

select l.city,l.latitude,l.longitude, 
st_distance_sphere(l.latlngindex, point(@lon, @lat)) as distance
from table_locations l
where st_within(l.latlngindex, @polygon)
order by distance 
;

It runs fine, gives result, but only first 5 or 6 are accurate wrt distance between them. Rest of them are inaccurate, I verified this on a couple of sites.

Table structure-

 `locationid`,
 `latitude`,
 `longitude`,
`latlngindex` point not null,
 spatial index `latlngindex` (`latlngindex`)

Sample insertions :

insert into `table_locations` values(2001,31.2372705, 121.4705291, Point(121.2372705, 31.4705291));
insert into `table_locations` values(2002,31.2328741, 121.4741493, Point(121.2328741, 31.4741493));
insert into `table_locations` values(2003,31.2300200, 121.4749245, Point(121.2300200, 31.4749245));
insert into `table_locations` values(2004,31.2302308, 121.4705508, Point(121.2302308, 31.4705508));
insert into `table_locations` values(2005,31.2391562, 121.4771425, Point(121.2391562, 31.4771425));
insert into `table_locations` values(2006,31.2331857, 121.4779539, Point(121.2331857, 31.4779539));

Sample result row :

   Lat          Long             distance
  31.2397267', '121.4742061', '35019.00977766075'

Distance between (31.2397267, 121.4742061) and (31.2304321, 121.4732134) should be 1004m, while it gives it to be 35019m.

Answer 1

Looks like you mixed up lat and lon

DROP TABLE IF EXISTS `table_locations`;

CREATE TABLE `table_locations` (
    `id` INT(11) NOT NULL AUTO_INCREMENT,
    `latitude` DOUBLE NOT NULL,
    `longitude` DOUBLE NOT NULL,
    PRIMARY KEY (`id`)
);

INSERT INTO `table_locations` VALUES (1, 31.2396691, 121.4798393);
INSERT INTO `table_locations` VALUES (2001, 31.2372705, 121.4705291);
INSERT INTO `table_locations` VALUES (2002, 31.2328741, 121.4741493);
INSERT INTO `table_locations` VALUES (2003, 31.2300200, 121.4749245);
INSERT INTO `table_locations` VALUES (2004, 31.2302308, 121.4705508);
INSERT INTO `table_locations` VALUES (2005, 31.2391562, 121.4771425);
INSERT INTO `table_locations` VALUES (2006, 31.2331857, 121.4779539);
INSERT INTO `table_locations` VALUES (2007, 31.2397267, 121.4742061);


SET @lat = 31.2304321;
SET @lon = 121.4732134;
SET @p = point(@lon, @lat);
SET @r = 43.0;
SET @POLY = ST_Buffer(@p, @r);

SELECT
      id
    , latitude
    , longitude
    , ST_Distance_Sphere(POINT(longitude, latitude), @p) as dist
FROM
    table_locations
WHERE st_within(POINT(longitude, latitude), @POLY)
ORDER BY
    dist
;

Returns values and shows distance 1204.9090584034252 for the first location