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I created an own map for my p&p rpg group and now I want to transfer it into QGis. That's easy to make, but I'd like to have an own CRS for that map but don't know how to use the right proj.4 definitions. Maybe anyone here can help me with that? The map will be 30.000km from West to East and 21.000km from north to South.

David

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Why do you need a CRS? If you don't set one in Qgis then Qgis will just assume your coordinates are on a plane. You won't be able to do things like convert them to lat-long, but then your world isn't on our world!

Is that 30-thousand km or 30-decimal-zero km?

If 30 km, then you could just keep your coordinates in metres and use British Ordnance Survey Grid references, and then your world would be a little square off the south-west coast of England if you tried to project it to a real map. But you don't need to give it a CRS unless you want to work with other data with other CRS systems, which I don't think you do.

Or... is this an entire planet, and you want it to be spherical, and you want to calculate distances between thing along great circles? Then you'd need to define a CRS with a different radius (or elliptical radius and eccentricity).

But otherwise, don't bother.

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It's 30000km and I need a CRS to measure distances, for nothing else. –  David Feb 12 '12 at 11:40
    
Not if your world is a plane. Just do sqrt(dx^2+dy^2). If you do anything in Qgis without setting the project CRS it will assume a cartesian planar system. –  Spacedman Feb 12 '12 at 12:17
    
So where do I put sqrt(dx^2+dy^2) ? We didn't really learn much about the use of that stuff at the university, only how to do things with layers and attributes etc. :-/ –  David Feb 12 '12 at 12:45
    
That's just the distance formula that a GIS should use when computing distances if you don't tell it what kind of sphere things are on by not setting a CRS. If you measure a distance between two points in Qgis without setting a CRS it'll give you the distance in whatever units your data are in, and based on it being a flat plane. –  Spacedman Feb 12 '12 at 13:43
    
ah okay. So I don't need an own CRS at all, that's good to know! Thank you very much for your help! :-) –  David Feb 12 '12 at 14:02
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