On 2013-03-14 23:18, Grant Slater wrote:
> On 14 March 2013 21:17, Steve <[email protected]> wrote: > >> Hello fellow OSMers A hopefully straightforward newbie question for you... I'm currently uploading various new polygons via shapefiles created in Quantum GIS. These use the WGS84 (EPSG:4326) projection, which Quantum defines as: +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +towgs84=0,0,0 A fellow mapper is doing similar, great stuff mapping administrative boundaries (Parishes etc) from Ordnance Survey, and setting the projection with the following parameters: +proj=longlat +ellps=WGS84 +datum=WGS84 +zone=30 +no_defs Because of the different parameters (i.e. I have +towgs84=0,0,0, and he has +zone=30), there is a slight offset in data we are creating in the same areas. The question is: which one of us is correct? > > Ordnance Survey data is (normally) EPSG:27700 > OpenStreetMap is: EPSG:4326 > > Zone is used for creating an offset origin point. Doubt that is what > you want... It sounds like the (incorrect) use of a "zone" value would create an error of the order of many degrees, not a few metres as Steve and I are seeing. The offset between his and my line is mostly a matter of 5m or so; "my" line is further north. If it wasn't for the fact that our lines are juxtaposed in a particular area, the difference might not even be noticeable to the "untrained eye" (like mine). Is it possible that the +zone=30 is being ignored because it is not specifying +proj=utm? The zone business is typical UTM isn't it? I got the Colin
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