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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