[postgis-users] making a circle with radius in meters, and lat/lon position

[Obe, Regina]

Antoine,

2163 (US National Atlas is an approximation of meters for Continental US).  If you use it for anywhere else, it will become very inaccurate

To get better answers, use UTM zone 

Use the UTMzone function http://postgis.refractions.net/support/wiki/index.php?plpgsqlfunctions

in wiki to determine right SRID for region.  So would be

SELECT ST_Transform(ST_Buffer(foofoo.the_geom_utm ,100), 4326) As buffer_geom,
                      ST_distance_sphere(ST_Transform(ST_Centroid(ST_Buffer(foofoo.the_geom_utm ,100)),4326),ST_Transform(ST_StartPoint(ExteriorRing(ST_Buffer(foofoo.the_geom_utm ,100))),4326))
As dist_meters
FROM 
(SELECT foo.the_geom, ST_Transform(foo.the_geom, utmzone(foo.the_geom) ) As the_geom_utm
FROM 
(SELECT ST_GeomFromText('POINT(0.0 20)',4326) As the_geom) As foo) As foofoo

---

Distance result = 99.76 meters
 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Lucas Antoine (External)
Sent: Wednesday, October 22, 2008 3:24 AM
To: 'PostGIS Users Discussion'
Subject: RE: [postgis-users] making a circle with radius in meters, and lat/lon position

Hi,

Indeed, making a buffer in my SRID (4326) will not makes a perfect circle.

But how to make a circle then ?

area = transform(Buffer(transform(GeomFromText('POINT(0.0 20)',4326),
                           2163) ,100), 4326)

should be the correct function (point in lat / lon, transformed in meters
then circle, then back in lat lon).

but then distance between a point of circle and the center:

Distance( transform(ST_Centroid(area),2163),
          transform(PointN(ExteriorRing(area),1),2163))

return 100 meters

distance_sphere(ST_Centroid(area),PointN(ExteriorRing(area),1))

return 127 meters.

And also, when I get all points and try to compute radius by hand, I get
again 127 meters.

Why distance_sphere and Distance function do not return same result ?

Antoine.

-----Message d'origine-----
De : postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net]De la part de
Michael Smedberg
Envoyé : mardi 21 octobre 2008 22:30
À : PostGIS Users Discussion
Objet : RE: [postgis-users] making a circle with radius in meters, and
lat/lon position


Hey Lucas,

I'm not sure I 100% understand your question, but I think that you're
making an incorrect assumption.  When you " transform distance into
angle and make buffer directly in my SRID (4326)", I don't think that
the result will be a circle.

For instance, when you're near the North Pole, a degree of longitude is
shorter than a degree of latitude.  If you were, say, 2 degrees south of
the North Pole, and you drew a polygon around you that was one degree
away, the result would not be a circle, and would be shorter (as
measured in meters) in the East-West direction than in the North-South
direction, right?

In other words, I don't think you can draw a circle in 4326- a "circle"
in degrees isn't a circle in actual distance.

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Lucas Antoine (External)
Sent: Monday, October 20, 2008 12:35 AM
To: 'postgis-users at postgis.refractions.net'
Subject: [postgis-users] making a circle with radius in meters,and
lat/lon position

Hi all,

I am using postgis 1.2.1 and I would like to make a circle. I use meters
unit and lat/lon coordinate in SRID 4326.

I saw two solutions:
  * transform point to a meter-SRID 2163 then make a buffer in meters
and
back transform in my SRID (4326)
  * transform distance into angle and make buffer directly in my SRID
(4326)

To check result, I first watch all coords, then recompute center, and
compute distance between center and a circle point.

With function Distance: OK
with function distance_sphere: 27% too much !

I made some verification by hand and I find a result close to
distance_sphere; where is the mistake ?

Here is a sample:

-- SQL file
create table toto (id int4 );

SELECT ADDGEOMETRYCOLUMN('public', 'toto', 'area', 4326, 'GEOMETRY', 2);


-- insert a circle with 100 meter radius
--
-- with transform function
--
insert into toto (area) values (transform(
                                    Buffer(
                                        transform(
                                            GeomFromText('POINT(0.0
20)',4326), -- point in lat lon
                                            2163) -- point in meters
coords
                                        ,100), -- circle in meters
coords
                                    4326) );   -- circle in lat / lon

-- insert a circle with 100 meter radius
--
-- transform directly 100 meters to angle (approximation from 4326 model
?)
--
insert into toto (area) values ( Buffer(GeomFromText('POINT(0.0
20)',4326),100.0/(6376500.0*3.14159*2.0)*360.0  ) );

--
-- distance with transform function (coherent with transform circle)
--
select Distance( transform(ST_Centroid(area),2163),
                 transform(PointN(ExteriorRing(area),1),2163)) from
toto;

     distance
------------------
  99.9998086273481  -> ok
 102.031862045723   -> ok
(2 rows)

--
-- distance with transform function (not coherent with transform circle)
--
select distance_sphere(ST_Centroid(area),PointN(ExteriorRing(area),1))
from
toto;
 distance_sphere
------------------
 127.779274484879   -> no ok !
 93.8880956491054
(2 rows)
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