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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.

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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.

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I would use QGIS or Global Mapper to easily convert the KML from EPSG:3857 (which is the id of WGS84 Web Mercator) See: Here

to EPSG:27700 (which is the id of the British National Grid on which OS is based, see Here).

After you are done with this you can convert your new KML to SVG and you will have way less trouble to fit those boundaries over the OS map you already have.

enter image description here

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  • Hi John and also @Andrejoost. I have downloaded qgis and have attempted to convert the .kml through Raster > Projections > Warp (Reproject). Firstly is this the correct method? Secondly, when I choose the .kml as the input file, i get an error saying the file format is not supported. I have tried with all other file formats the website allowed you to download including .geojson and .wkt with no success. This is the website I am downloading these boundaries from if it helps: mapit.mysociety.org/area/65982.html. Thank you very much for your help! – John Larry Aug 11 '13 at 14:47
  • Hi Larry. I have downloaded the KML file and imported it to QGIS without any problem. Its original EPSG was 4326 as one could expect and I converted it to osgb1936 (epsg:27700) for you. You can find the link to the converted shapefile here: thgis.hu/so_johnlarry.zip and to the converted svg here: thgis.hu/so_johnlarry.svg – EZMapdesign Aug 11 '13 at 20:43
  • I have also attached a screenshot where I indicated where you can change the projection of the project. If you try to do the process yourself, after reprojecting, right click the layer in the layer pane, click Save as and select your CRS from the list (you need EPSG:27700) and there you go! – EZMapdesign Aug 11 '13 at 20:46

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