[Talk-GB] OSM Analysis New Data and bot

[SK53 on OSM]

On 14/06/2011 10:41, Craig Loftus wrote:
> On 14 June 2011 10:26, Richard Fairhurst<richard at systemed.net>  wrote:
>> Potlatch 2's improved (not deployed yet) shapefile background layers asks
>> you for the projection before loading, rather than relying on the .prj file.
> Thanks for clarifying.
>
> I'm not very familiar with projections so a little more clarity would
> be useful for me. As potlatch 'understands' OSGB, is loading an OSBG
> shp file any more expensive than loading a WGS84 shp file?
>
> If it is, it may be worthwhile reprojecting all the files. This would
> also have the advantage of making the mirror useful for those of us
> who simply refuse to stop using JOSM, despite all the awesome you keep
> piling into potlatch.
>
> Craig
>
> _______________________________________________
> Talk-GB mailing list
> Talk-GB at openstreetmap.org
> http://lists.openstreetmap.org/listinfo/talk-gb
JOSM is perfectly capable of dealing with different projections: it's 
how the French cadastre plugin works.

Adding an OSGB36 capability to JOSM would seem to the way to go.

There is a basic rule about shapefiles and OSM imports: learn about 
projections first!

RichardF's mail shows why: if you dont add all the funny numbers at the 
end of the projection (the Helmert Transform) things end-up 100m away 
from where they should be! There's a good intro on the OS site, also 
Chris Hill wrote a blog post about this last April or May.

Jerry