I am currently in the process of creating a map with some administrative boundaries on it. The base map I am using is an ordnance survey map 1:10000. I downloaded an administrative border as a .kml and converted into an .svg file. When I tried to overlay this border on top of the ordnance survey map, it just wouldn't fit.
I slowly realised that this must be because of different projections between the ordnance survey map and the .kml which was obviously based on a Google Map.
Could someone point me in the right direction of how to warp the boundary overlay so it will fit the map?
I am using Photoshop to create this map so I apologise in advance if this is the wrong website to ask for help.
Or if someone could tell me how the ordnance survey grid compares to the Google Maps grid shape-wise (is it a trapezoid or an arc, etc.), I could try and warp it through trial and error.
Best Answer
Coordinates in kml files are usually in lat/long degrees. You can open the file with a text editor to check sample data points. So the coordinate system should be EPSG:4326 WGS84 (not WGS84 Web Mercator) .
Your Ordnance Survey data will probably have metres as units.
Degrees get smaller towards the poles. That's why you can not easily warp the kml to a projected map with photoshop.
You need a reprojection from WGS 84 to British OS. I would use Quantum GIS for that. You can also add a Google background with the openlayers plugin.