[postgis-users] Problem with mismatched coordinate systems

[Fonseca Hespanha de Oliveira, Joao da]

Russell:

I do not know about the coordinate systems in use in UK, so I do not
know the reason why you reverted the place names projected coordinates
to geographic coordinates and what was their original coordinate system.
Anyhow, I see a problem on using just three decimal positions on the
decimal degrees values for lat and long. Just think that each arcsecond
corresponds to roughly 30 meters on the ground! You should use some
transformation that can give  seven or even eight decimal positions for
the lat and long values.
Or try to use the projected coordinates from the second data set, if the
coordinate system is known to EPSG.

Regards,
Joao

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Russell Horn
Sent: Wednesday, November 21, 2007 5:21 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Problem with mismatched coordinate systems

Hi,

I am very new to PostGIS, please bear with me. I've had a lot of help
from #postgis on IRC but I continue to have trouble and hope someone
here can assist.

I have two data sets from the UK Ordnance Survey which when imported to
PostGIS and plotted are not matching up:

http://exiled.albanach.com/chester.png

One data set contains shape files (no .prj file) which give me an
outline of part of the UK.

The data set is available here:

http://www.ordnancesurvey.co.uk/products/boundaryline/sampledata/Layered
_Shape.zip

The documentation states that the coordinate system is National Grid
(NG). I understand this is OSGB1936 / EPSG:27700

The other data set contains a list of place names, together with
Eastings and Northings like this:

CHESTER*CHESHIRE COUNTY*340363*366880

I have then converted that to Long Lat to give me

53.196,-2.894,CHESTER

I can paste my Long Lat values into Google maps and get the right place,
so that conversion seems to have worked.

To import my shape files I did this:

shp2pgsql -s 27700 county_region.shp public.ch_county > /tmp/county.sql

To import my place names, I did this:

ogr2ogr -f "ESRI Shapefile"  gaz/ chester.csv

Where chester.csv looks like this

lat,long,value
53.196,-2.894,CHESTER

I then did

ogr2ogr -f "ESRI Shapefile"  gaz/ chester.csv ogr2ogr -f "ESRI
Shapefile" gaz/ os.vrt

To make my shapefiles where os.vrt looks like this:

<OGRVRTDataSource>
  <OGRVRTLayer name="OS_Gazetteer">
    <SrcDataSource relativeToVRT="1">gaz</SrcDataSource>
    <SrcLayer>chester</SrcLayer>
    <GeometryType>wkbPoint</GeometryType>
    <LayerSRS>EPSG:27700</LayerSRS>
    <GeometryField encoding="PointFromColumns" x="long" y="lat"/>
  </OGRVRTLayer>
</OGRVRTDataSource>

Changing LayerSRS value didn't seem to change anything.

I converted to sql like this:

shp2pgsql OS_Gazetteer.shp public.chesgaz > gaz.sql

I then imported my two SQL files into Postgres.

When  plot my data using qgis I end up with Chester being plotted way
off my map like the image I pointed to earlier:

http://exiled.albanach.com/chester.png

I know I'm doing something wrong here, but I'm at a bit of a loss as to
what it is, and where to go from here.

Any help would be very much appreciated.

Thanks,

Russell
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