[postgis-users] Transforming Coordinates

Thu Jul 19 14:06:08 PDT 2007

Alan,

Sorry I didn't see this email until now.  Still not quite clear your
question, so hopefully the explanation below will help.

1) You can either statically store the values in the table doing
    UPDATE bus_stops1 SET xcoord = x(transform(east_north, 4326)),
ycoord = y(transform(east_north,4326))

where xcoord and ycoord are numeric or float fields you create to store
the values.

Then for inserting you would do
INSERT INTO bus_stops1(east_north, xcoord, ycoord)
    values(transform(setsrid(makepoint([lon],[lat]), 4326), 27700),
[lon], [lat])

where [lon] and [lat] are the x and y coords you get from google map.

2) Or alternatively as I mentioned you would create a view and for most
purposes except for inserting ignore your original table.  This is a bit
slower, but I don't think significantly slower.

---Now some explanation

x is a function that returns the x coordinate of a postgis point, y is a
function that returns the y coordinate of a postgis point geometry

So what I am doing here is first transforming to WGS 84 and then getting
the x and y.  Since the dimensions of the transformed geometry are in
degrees for 4326, then the x and y returned will be in degrees as well.

Hope that helps,
Regina

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Alan
Cunnane
Sent: Wednesday, July 18, 2007 4:49 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Transforming Coordinates

What I need to do is create a new column in the bus_stops1 table with x
and x coordinates. If I use your suggestion below to transform the
column east_north to x and y wont both the new X and Y columns be the
same value as it is the same query?

----- Original Message ----
From: "Obe, Regina" <robe.dnd at cityofboston.gov>
To: PostGIS Users Discussion <postgis-users at postgis.refractions.net>
Sent: Wednesday, 18 July, 2007 9:38:29 PM
Subject: RE: [postgis-users] Transforming Coordinates

Not quite sure what you are asking.  For this kind of stuff, I usually
create a view and use that for google maps if I have a lot of apps that
need it or you could alternatively just write the sql of the view each
time you need it. 

Would be something like this

CREATE VIEW vwbusstopsgoogle As
    select x(transform(east_north, 4326)) As lon,
y(transform(east_north,4326)) As lat, [other fields you need go here]
        FROM bus_stops1

The to use
SELECT lon, lat from vwbusstopsgoogle

To transform back it would be

INSERT INTO bus_stops1(east_north)
    values(transform(setsrid(makepoint([lon],[lat]), 4326), 27700))

WHERE [lon], [lat] you replace with the x y you get from google.

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Alan
Cunnane
Sent: Wednesday, July 18, 2007 4:28 PM
To: PostGIS Users Discussion
Subject: [postgis-users] Transforming Coordinates

Hi guys

I have a table with easting northing coordinates and these points in a
geom column. I have 15,000 points with an easting and a northing
(brittish national grid) and created the geom column using these
commands:

SELECT AddGeometryColumn( 'bus_stops1', 'east_north', 27700, 'POINT', 2
);
UPDATE bus_stops1 SET east_north = PointFromText('POINT(' || easting ||
' ' || northing || ')',27700);

The table looks like this at the moment:

 easting | northing |                     east_north
---------+----------+---------------------------------------------------
-
  226965 |   676038 | 0101000020346C000000000000A8B40B41000000008CA12441
  226761 |   675945 | 0101000020346C00000000000048AE0B4100000000D2A02441
  226696 |   675893 | 0101000020346C00000000000040AC0B41000000006AA02441
  226473 |   675777 | 0101000020346C00000000000048A50B4100000000829F2441
  226465 |   675731 | 0101000020346C00000000000008A50B4100000000269F2441
  226614 |   675611 | 0101000020346C000000000000B0A90B4100000000369E2441
  226603 |   675605 | 0101000020346C00000000000058A90B41000000002A9E2441
  226869 |   675621 | 0101000020346C000000000000A8B10B41000000004A9E2441
  226883 |   675630 | 0101000020346C00000000000018B20B41000000005C9E2441
  226544 |   674911 | 0101000020346C00000000000080A70B4100000000BE982441
  226767 |   675293 | 0101000020346C00000000000078AE0B4100000000BA9B2441
  226767 |   675303 | 0101000020346C00000000000078AE0B4100000000CE9B2441

Now what I want to be able to do is to add another column or two columns
with X and Y values in WGS or the standard X and Y coordinates for entry
into google maps. I dont want to transform the east_north column so that
I get another long string as is shown above as I cannot then integrate
them into google maps. Can this be done using PostgreSQL?

Also I would like to do the opposite too. Take x and y coordinates from
google maps and transform them to easting northing as above. Thanks for
your help again

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