[Talk-GB] OS Boundaries

[Jerry Clough - OSM]

Apologies for top posting.

I've been experimenting with the
Civil Parish boundaries with QGIS and ogr2osm.py (modified to keep ways
below 500 nodes). QGIS seems rather temperamental about reprojecting
shape files: there are a huge number of settings which seem to affect
the base projection. However, I have managed to use QGIS to select
sub-sets of civil parishes and save them using the same projection as
the source data. These smaller data sets are much easier to use for
testing. 

The standard projection information for the OSGB shape files is contained in .prj and is in Well-known Text format. However, it lacks the Helmert data transformation parameters recommended by the OS at http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide6.html.
Editing the .prj file to add these (see the example on the wikipedia
link, but the parameters given there are incorrect), makes a big
difference to the .OSM output. Note these parameters still only give an
accuracy in the order of 5m, but they do seem to allow GDAL-based
utilities to be used.

Here's the format used, the addition being the TOWGS84 section:

PROJCS["British
National Grid (ORD SURV GB)", GEOGCS["unnamed", DATUM["D_OSGB_1936",
SPHEROID["Airy - 1848",6377563,299.319997677743],
TOWGS84[446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894]],
PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"], PARAMETER["latitude_of_origin",49], 
PARAMETER["central_meridian",-2],
PARAMETER["scale_factor",0.9996012717], 
PARAMETER["false_easting",400000], PARAMETER["false_northing",-100000],
UNIT["METER",1]]

Jerry




________________________________
From: Richard Fairhurst <richard at systemeD.net>
To: talk-gb at openstreetmap.org
Sent: Tue, 20 April, 2010 10:33:51
Subject: Re: [Talk-GB] OS Boundaries

Andrew Chadwick wrote:

> I've had a degree of success with
> http://search.cpan.org/~toby/Geo-Coordinates-OSGB-2.04/ - I've used  
> these packages in the past for rectification of OOC OS stuff and  
> conversion of many-figure OS grid refs with a good degree of success.

Chris knows this already (because we talked about it on IRC last  
night) but Matthew Somerville's proviso at  
http://www.systemed.net/blog/?p=182&cpage=1#comment-971 is worth  
taking into account.

cheers
Richard


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