我想问一下如何创建一个圆radius=4km 。我曾尝试ST_Buffer功能，但它创造一个更大的圈子。（我看通过将其多边形到一个新的KML文件创建的圈子。）

这就是我努力。

INSERT INTO camera(geom_circle) VALUES(geometry(ST_Buffer(georgaphy(ST_GeomFromText('POINT(21.304116745663165 38.68607570952619)')), 4000)))

圆的中心是一个经度纬度点，但我不知道它的SRID ，因为我已经从一个KML文件导入它。我需要的SRID以改造几何等？

KML文件总是纬度/长，使用SRID = 4326。如果您使用此SRID是隐含的geography 。地理是混合式4公里的度量指标上拉/长数据的好办法...优秀您是否尝试过这个！

试试这个语句修复了石膏，并使用参数的构造函数点：

SELECT ST_Buffer(ST_MakePoint(21.304116745663165, 38.68607570952619)::geography, 4000);

如果你需要投这回的几何形状，增加::geometry投进行到底。

更新的准确性

以前的答案内部重新项目的几何形状（通常情况下），以该点适合内的UTM区（见ST_Buffer ）。这可能会导致轻微的失真，如果点是在两个边界UTM的边缘。大多数人不会在乎这些错误的大小，但它往往是几米。但是，如果您需要亚毫米精确度，可考虑建立一个动态的等距方位投影。这需要PostGIS的2.3的ST_Transform ，并从适应另一种答案：

CREATE OR REPLACE FUNCTION geodesic_buffer(geom geometry, dist double precision,
                                           num_seg_quarter_circle integer)
  RETURNS geometry AS $$
  SELECT ST_Transform(
    ST_Buffer(ST_Point(0, 0), $2, $3),
      ('+proj=aeqd +x_0=0 +y_0=0 +lat_0='
       || ST_Y(ST_Centroid($1))::text || ' +lon_0=' || ST_X(ST_Centroid($1))::text),
      ST_SRID($1))
  $$ LANGUAGE sql IMMUTABLE STRICT COST 100;
CREATE OR REPLACE FUNCTION geodesic_buffer(geom geometry, dist double precision)
  RETURNS geometry AS 'SELECT geodesic_buffer($1, $2, 8)'
  LANGUAGE sql IMMUTABLE STRICT COST 100;
-- Optional warppers for geography type
CREATE OR REPLACE FUNCTION geodesic_buffer(geog geography, dist double precision)
  RETURNS geography AS 'SELECT geodesic_buffer($1::geometry, $2)::geography'
LANGUAGE sql IMMUTABLE STRICT COST 100;
CREATE OR REPLACE FUNCTION geodesic_buffer(geog geography, dist double precision,
                                           num_seg_quarter_circle integer)
  RETURNS geography AS 'SELECT geodesic_buffer($1::geometry, $2, $3)::geography'
  LANGUAGE sql IMMUTABLE STRICT COST 100;

一个简单的例子来运行的功能之一是：

SELECT geodesic_buffer(ST_MakePoint(21.304116745663165, 38.68607570952619)::geography, 4000);

而到了距离比较每个缓冲点，这里是每一个的长度测（上旋转椭圆最短路径，即WGS84）。首先这个功能：

SELECT count(*), min(buff_dist), avg(buff_dist), max(buff_dist)
FROM (
  SELECT ST_Distance((ST_DumpPoints(geodesic_buffer(poi, dist)::geometry)).geom, poi) AS buff_dist
  FROM (SELECT ST_MakePoint(21.304116745663165, 38.68607570952619)::geography AS poi, 4000 AS dist) AS f
) AS f;

 count |      min       |       avg       |      max
-------+----------------+-----------------+----------------
    33 | 3999.999999953 | 3999.9999999743 | 4000.000000001

与此相比，ST_Buffer（答案的第一部分），这表明它由约一点五六M的关闭：

SELECT count(*), min(buff_dist), avg(buff_dist), max(buff_dist)
FROM (
  SELECT ST_Distance((ST_DumpPoints(ST_Buffer(poi, dist)::geometry)).geom, poi) AS buff_dist
  FROM (SELECT ST_MakePoint(21.304116745663165, 38.68607570952619)::geography AS poi, 4000 AS dist) AS f
) AS f;

 count |      min       |       avg        |      max
-------+----------------+------------------+----------------
    33 | 4001.560675049 | 4001.56585986067 | 4001.571105793