Find the radius of circle in PostGIS using query in php?

Question

I have the geometry type defined for the polygons and circle in PostGIS and now I need to find the radius of the circle in this geometry type?? I have found the centre by using ST_X(ST_ASTEXT(ST_CENTROID(shape))) AS lat, ST_Y(ST_ASTEXT(ST_CENTROID(shape))) as lgt, Can anyone help me to get the radius ??

Answer 1

You can use the ST_Envelope function to get the bounding box of any polygon (not just a circle). You can then use the ST_XMin , ST_YMin , ST_XMax and ST_YMax functions to extract the width and height. As it is a circle, you could use either, so something like:

SELECT (ST_XMax(bbox)-ST_XMin(bbox))/2 as radius
FROM 
  (SELECT ST_Envelope(shape) as bbox from sometable) env 

where the sub-query is just shorthand to avoid having to call ST_Envelope twice, but you could also write,

SELECT (ST_XMax(ST_Envelope(shape)) - ST_XMin(ST_Envelope(shape)))/2 as radius
FROM sometable

which seems shorter, but, is I feel less elegant, and would be longer if you wanted the height. The query optimizer will view them as the same query, either way.