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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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: [EMAIL PROTECTED] To: [email protected] 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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Sinboy Chang Sent: Wednesday, March 19, 2008 3:09 AM To: [email protected] 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? _____ The substance of this message, including any attachments, may be confidential, legally privileged and/or exempt from disclosure pursuant to Massachusetts law. It is intended solely for the addressee. If you received this in error, please contact the sender and delete the material from any computer. _____ Help make the earth a greener place. If at all possible resist printing this email and join us in saving paper.
_______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
