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List: postgis-users
Subject: [postgis-users] How inaccurate is Geometry
From: "Chen, Li [Contractor]" <Li.Chen () andrew ! com>
Date: 2010-04-28 23:34:39
Message-ID: 8FAF8DBABDE6D643926E35A54B67D02033D9122CD6 () SISPE7MB2 ! commscope ! com
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Hi Everyone,
I just want to ask a question regarding Geometry data type.
According to the manual, geometry use a planar coordinate system for measurement and \
the geography data type use a geodetic coordinate system.
In my application, I need to deal with data from all over the world and I can not use \
geography because I do need ST_Covers/ST_Intersects. For geography type, ST_Covers \
only supports polygon covers point and ST_Intersects is really slow (~5 sec for one \
query). Therefore, I have to use geometry type.
However, I also notice in the manual says that geometry will auto cast to geography \
if I use SRID=4326. I did specify SRID to be 4326 when I create all the geometry \
objects.
So here is my question, what sort of inaccuracy/accuracy(in terms of meters) can I \
expect if I specify a circle ('POLYGONG(x x, xx,xx)') around 1000km radius( I use \
polygon to approximate circle)? What if I use ST_buffer to create this circle, does \
it make any difference in terms of accuracy?
I tried to create two geometry points and compare the distance between them with two \
geography points have the same locations using ST_Distance(geog, geog) and \
ST_Distance_Sphere(geom, geom), ST_Distance_Spheroid(geom, geom). Although \
ST_Distance returns some difference from ST_Distance_Sphere, the results for \
ST_Distance and ST_Distance_Spheroid is exactly the same. So I guess this could not \
tell the difference between geom and geog, rather it is a matter of functions used.
Thanks for any suggestions,
Li
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<p class=MsoNormal>Hi Everyone,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I just want to ask a question regarding Geometry data \
type.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>According to the manual, geometry use a planar coordinate
system for measurement and the geography data type use a geodetic coordinate
system.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>In my application, I need to deal with data from all over
the world and I can not use geography because I do need
ST_Covers/ST_Intersects. For geography type, ST_Covers only supports polygon
covers point and ST_Intersects is really slow (~5 sec for one query).
Therefore, I have to use geometry type.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>However, I also notice in the manual says that geometry will
auto cast to geography if I use SRID=4326. I did specify SRID to be 4326 when I
create all the geometry objects.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>So here is my question, what sort of inaccuracy/accuracy(in
terms of meters) can I expect if I specify a circle (‘POLYGONG(x x,
xx,xx)’) around 1000km radius( I use polygon to approximate circle)? What
if I use ST_buffer to create this circle, does it make any difference in terms
of accuracy?<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I tried to create two geometry points and compare the
distance between them with two geography points have the same locations
using ST_Distance(geog, geog) and ST_Distance_Sphere(geom, geom),
ST_Distance_Spheroid(geom, geom). Although ST_Distance returns some difference
from ST_Distance_Sphere, the results for ST_Distance and ST_Distance_Spheroid
is exactly the same. So I guess this could not tell the difference
between geom and geog, rather it is a matter of functions used.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Thanks for any suggestions,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Li <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
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