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 <[email protected]> To: [email protected] 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 _______________________________________________ Talk-GB mailing list [email protected] http://lists.openstreetmap.org/listinfo/talk-gb
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