Saturday, 3 April 2010

Reprojecting Ordnance Survey Raster Data

I have been looking at the Ordnance Survey StreetView data that was released to the public on 1st April.
This has got me thinking about a previous project that I failed to make work - I have some old maps of my home town and wanted to be able to display them on the web and switch between them so you can see what changes from year to year - I have maps from 1870 and 1914.  I want to use OpenStreetMap as the recent data.  Now StreetView is available I can use that too for comparison.

The problem is projecting them onto the same coordinate system - I failed miserably last time, but my two scanned old maps were on the same scale, so you can see them at (http://maps.webhop.net/oldmaps/openlayers.html).

The maps are in Ordnance Survey projection, which has the code EPSG:27700.   The downloaded OS data has the origin specified in metres northings and eastings.
You can translate this into the same projection as OpenStreetMap (EPSG:900113) using:
gdalwarp -s_srs EPSG:27700 -t_srs EPSG:900913 *.TIF nz.tif
Check the projection information with
gdalinfo nz.tif | more

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