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

[R H]

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.usTo: postgis-users at postgis.refractions.netSubject: 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 HSent: Wednesday, March 19, 2008 6:03 PMTo: PostGIS Users DiscussionSubject: 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 -0400From: robe.dnd at cityofboston.govTo: 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 ChangSent: Wednesday, March 19, 2008 3:09 AMTo: postgis-users at postgis.refractions.netSubject: [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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