[postgis-users] How to get the distance between geometry inmeters?

[David William Bitner]

Distance() always returns "cartesian distance" in whatever units it is fed.
If it is fed data that is in meters -- the output will be in meters.  If it
is fed degrees -- the output will be in degrees and totally useless.  I
don't quite understand the problem that you are having

On Thu, Mar 20, 2008 at 11:15 AM, R H <enoop at hotmail.com> wrote:

>  Thanks for the reply.  I think I got it figured out.  The projections I
> was putting into distance() where in meters.  I used a projection that is in
> degrees and then transformed to meters.  It appears to be working now.
>
> I'm not sure I understand why?
> This is what I had previously
> geom = sird(26912) meter projection
>
> select distance(geom,geom) returns cartesian distance
> select distance(transform(geom, 26912), transform(geom, 26912)) returns
> cartesian distance
>
> then I changed geom = sird(4269) degree projection
> select distance(transform(geom, 26912), transform(geom, 26912)) returns
> meter
>
> How are any of these statements different?  Based on order of operations
> isn't the inner most parentheses executed first?  Therefore being exactly
> the same as the first select?  I guess it's working so I will stop worrying
> about it.
>
> ------------------------------
> From: lr at pcorp.us
> To: postgis-users at postgis.refractions.net
> Subject: RE: [postgis-users] How to get the distance between geometry
> inmeters?
> Date: Thu, 20 Mar 2008 05:37:30 -0400
>
>
> RH,
>
> Yes this is the same as putting 2 geoms with the *same *meter projection
> into ST_Distance.  The transform effectively flattens out a small section of
> space so you are measuring in cartesian space which is actually what you
> want to do since it simplifies the math.
>
> Your example below transform(st_distance(geom,geom),srid)
> should give you an error since you are effectively taking a distance
> (which would be in double precision) and then trying to transform that.  You
> can only transform geometries not distances.
>
> What sort of error are you getting with ST_Distance and ST_DWithin?  Keep
> in mind that both your geometries have to be in the same meter projection
> otherwise an error would be thrown.
>
> Hope that helps,
> Leo & Regina
>
>
>
>  ------------------------------
> *From:* postgis-users-bounces at postgis.refractions.net [mailto:
> postgis-users-bounces at postgis.refractions.net] *On Behalf Of *R H
> *Sent:* Wednesday, March 19, 2008 6:03 PM
> *To:* PostGIS Users Discussion
> *Subject:* RE: [postgis-users] How to get the distance between geometry
> inmeters?
>
> isn't this the same as putting two geom w/ a meter projection into
> st_distance(), st_distance() still returns a cartesian distance not a
> distances in meters?  Wouldn't you need to do something like...
> transform(st_distance(geom,geom),srid)???
>
> I only ask because I am having a simular problem.  My projections are in a
> meter projection, but I can't get st_distance() or st_dwithin() to use
> meters?  any thoughts?
>
>
>  ------------------------------
> Subject: RE: [postgis-users] How to get the distance between geometry in
> meters?
> Date: Wed, 19 Mar 2008 06:56:24 -0400
> From: robe.dnd at cityofboston.gov
> To: postgis-users at postgis.refractions.net
>
> You'll need to transform to a meter based projection to do that.  If for
> example you are in the U.S, SRID 2163 US National Atlas Equal Area works
> pretty good for measurement.  It is not as accurate as UTM or State Plane,
> but its pretty good and covers all of US I believe and probably works for
> some of Canada as well (not sure about non-continental e.g. Hawaii,
> Alaska).
>
>  e.g.
>
> SELECT road.rd_name, ht.ht_name, ST_Distance(ST_Transform(roads.the_geom,2163)
> , ST_Transform(hotel.the_geom,2163))  as dist_meters
> FROM road, hotel
> WHERE <some criteria here>
>
> If you are somewhere else and your locations are within that projection,
> you can use that.  Look at the utmzone function in the wiki that will help
> you determing the right UTM zone SRID for your data
>
> http://postgis.refractions.net/support/wiki/index.php?plpgsqlfunctions
>
> So you would use something liek
> SELECT utmzone(ST_Centroid(the_geom)) as srid
>
> to get the right UTM zone SRID
>
>
> Hope that helps,
> Regina
>
>
>
>  ------------------------------
> *From:* postgis-users-bounces at postgis.refractions.net [mailto:
> postgis-users-bounces at postgis.refractions.net] *On Behalf Of *Sinboy Chang
> *Sent:* Wednesday, March 19, 2008 3:09 AM
> *To:* postgis-users at postgis.refractions.net
> *Subject:* [postgis-users] How to get the distance between geometry in
> meters?
>
>  i want to get the distance between geometry,just like road or
> hotel poi,but ST_Distance() return the result in *cartesian dintance.*
> So how can get the distance() by meters?
> ------------------------------
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-- 
************************************
David William Bitner
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