[postgis-users] Distance query not working after transfrom?

[Burgholzer,Robert]

What I typically do, is use the transform function in my distance query
to bring it to something that uses meters, so it goes like:

 

Select distance(transform(a.the_geom, 27700), transform(b.the_geom,
27700)) 

...

 

I may be totally off with this approach, there may be some better way of
telling PostGIS what units you want, but this works for me, and is
verified by comparison to the original shapefiles (if ArcFoo can be
considered as verification:-) )

 

HTH,

r.b.

 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Alan
Cunnane
Sent: Thursday, July 19, 2007 2:59 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Distance query not working after transfrom?

 

Could you suggest to me how I can the get the result in meters and not
degrees?  Thanks very much for your help

----- Original Message ----
From: "Burgholzer,Robert" <rwburgholzer at deq.virginia.gov>
To: PostGIS Users Discussion <postgis-users at postgis.refractions.net>
Sent: Thursday, 19 July, 2007 7:55:07 PM
Subject: RE: [postgis-users] Distance query not working after transfrom?

I think that this is an entirely correct result, as you are now getting
it in the new units, which are degrees, rather than meters.

 

r.b.

 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Alan
Cunnane
Sent: Thursday, July 19, 2007 2:51 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Distance query not working after transfrom?

 

Im having a problem with a distance query after I transformed a set of
points to another SRID. I transformed from easting northing (SRID 27700)
to longitude lattitude (SRID 4326).  The problem that I am having is
that when I do the same distance queries using both geom columns I get a
completely different result? I know that the distance query using the
first easting northing column is correct. Here is an example of the
different returns I am getting:

SRID (27700):

SELECT a.stop_reference, b.stop_reference, distance(a.east_north,
b.east_north) FROM bus_stops1 a, bus_stops1 b WHERE a.stop_reference =
6200206290 AND b.stop_reference = 6200244450;

stop_reference | stop_reference |     distance
----------------+----------------+------------------
 6200206290     | 6200244450     | 4320.88972319359

SRID (4326):

SELECT a.stop_reference, b.stop_reference, distance(a.googlemap,
b.googlemap) FROM bus_stops1 a, bus_stops1 b WHERE a.stop_reference =
6200206290 AND b.stop_reference = 6200244450;

stop_reference | stop_reference |     distance
----------------+----------------+-------------------
 6200206290     | 6200244450     | 0.041147965100606

Here is the table I am selecting from:


            Table "s0679212.bus_stops1"
     Column     |         Type          | Modifiers
----------------+-----------------------+-----------
 stop_reference | character varying(12) | not null
 easting        | integer               | not null
 northing       | integer               | not null
 full_location  | character varying(50) |
 gazetteer_code | character varying(1)  |
 point_type     | character varying(1)  |
 nat_gazetteer  | character varying(7)  |
 district_name  | character varying(24) |
 town_name      | character varying(24) |
 east_north     | geometry              | not null
 lat            | double precision      |
 lon            | double precision      |
 googlemap      | geometry              |
Indexes:
    "bus_stops1_pkey" PRIMARY KEY, btree (stop_reference)
    "stops_distance1" gist (east_north)
Check constraints:
    "enforce_dims_east_north" CHECK (ndims(east_north) = 2)
    "enforce_dims_googlemap" CHECK (ndims(googlemap) = 2)
    "enforce_geotype_east_north" CHECK (geometrytype(east_north) =
'POINT'::text OR east_north IS NULL)
    "enforce_geotype_googlemap" CHECK (geometrytype(googlemap) =
'POINT'::text OR googlemap IS NULL)
    "enforce_srid_east_north" CHECK (srid(east_north) = 27700)
    "enforce_srid_googlemap" CHECK (srid(googlemap) = 4326)

I created the googlemap column using these commands:

SELECT AddGeometryColumn( 'bus_stops1', 'googlemap', 4326, 'POINT', 2 );
UPDATE bus_stops1 SET googlemap = transform(setsrid(makepoint(easting,
northing),27700), 4326);


Can anyone suggest what on earth could be wrong?





 

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