<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:times new roman,new york,times,serif;font-size:12pt">Apologies for top posting.<br><br>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. <br><br>The standard projection information for the OSGB shape files is contained in .prj and is in <a rel="nofollow" target="_blank" href="http://en.wikipedia.org/wiki/Well-known_text">Well-known Text</a> format. However, it lacks the Helmert data transformation parameters recommended by the OS at <a rel="nofollow" target="_blank" href="http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide6.html">http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide6.html</a>.
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.<br><br>Here's the format used, the addition being the TOWGS84 section:<br><br>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]]<br><br>Jerry<div><br></div><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><br><div style="font-family: arial,helvetica,sans-serif; font-size: 13px;"><font face="Tahoma" size="2"><hr size="1"><b><span style="font-weight: bold;">From:</span></b> Richard Fairhurst <richard@systemeD.net><br><b><span style="font-weight: bold;">To:</span></b> talk-gb@openstreetmap.org<br><b><span style="font-weight: bold;">Sent:</span></b> Tue, 20 April, 2010 10:33:51<br><b><span style="font-weight: bold;">Subject:</span></b> Re: [Talk-GB] OS Boundaries<br></font><br>Andrew Chadwick wrote:<br><br>> I've had a degree of success with<br>> <a href="http://search.cpan.org/%7Etoby/Geo-Coordinates-OSGB-2.04/" target="_blank">http://search.cpan.org/~toby/Geo-Coordinates-OSGB-2.04/</a> - I've used <br>> these packages in the past for rectification of OOC OS stuff and <br>> conversion of many-figure OS
grid refs with a good degree of success.<br><br>Chris knows this already (because we talked about it on IRC last <br>night) but Matthew Somerville's proviso at <br><a href="http://www.systemed.net/blog/?p=182&cpage=1#comment-971" target="_blank">http://www.systemed.net/blog/?p=182&cpage=1#comment-971</a> is worth <br>taking into account.<br><br>cheers<br>Richard<br><br><br>_______________________________________________<br>Talk-GB mailing list<br><a ymailto="mailto:Talk-GB@openstreetmap.org" href="mailto:Talk-GB@openstreetmap.org">Talk-GB@openstreetmap.org</a><br><a href="http://lists.openstreetmap.org/listinfo/talk-gb" target="_blank">http://lists.openstreetmap.org/listinfo/talk-gb</a><br></div></div>
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