[Talk-GB] Offsets between OS and OSM data (was Building with mapseg)

[Jerry Clough - OSM]

Hi Tim,

Yes there is, at least with the version of GDAL I use. Chillly has written about this on his blog, and the changes needed (adding Helmert transformations - -sound fancy doesn't it) to the standard projection are noted in previous messages here in talk-gb.

I think the divergence is much greater on the E of the country: probably why Chilly and I worried most about it. Even with these the accuracy compared with the OSGB02 will be upto 5 metres out. See OS Coordinate Systems Guide.

Jerry

PS. StreetView and OSM seem to match up quite well for Nottingham. I've just rendered a set of tiles in OSGB36 of the same scale and boundaries as StreetView which at least removes some of the projection transformation artefacts.





________________________________
From: Tim François <sk1ppy14 at yahoo.co.uk>
To: Talk-GB at openstreetmap.org; Chris Hill <osm at raggedred.net>
Sent: Sun, 30 May, 2010 12:16:33
Subject: Re: [Talk-GB] Offsets between OS and OSM data (was Building with mapseg)


By the way, which method are people using to re-project the VectorDistrict data? I'm using the inbuilt datum in gdal - is anyone using the correct *.prj file, and is there a difference?

Tim

--- On Sun, 30/5/10, Chris Hill <osm at raggedred.net> wrote:


>From: Chris Hill <osm at raggedred.net>
>Subject: Re: [Talk-GB] Offsets between OS and OSM data (was Building with mapseg)
>To: Talk-GB at openstreetmap.org
>Date: Sunday, 30 May, 2010, 11:53
>
>
>I believe the StreetView tiles are offset south(ish) by a few metres in 
>East Yorkshire too. Reprojected shape files line up well with surveyed 
>data. I have traced a few buildings from StreetView but I've stopped 
>until I had worked out what was
> wrong. Now given other people's comments 
>I do think there may be some discrepancy.
>
>Would a few carefully surveyed road junctions with many GPS traces to 
>work from help to identify any discrepancy? Or is there a better way?
>
>Cheers, Chris
>
>Kevin Peat wrote:
>> I'm in Devon and I see the same thing although whether it is just the 
>> SW I don't know. 
>>
>> The Streetview tiles (as I see them in JOSM) are all offset to the SE 
>> by 5-10 metres. I've converted some woods in my area from the 
>> VectorDistrict data using this process,
>>
>>     http://wiki.openstreetmap.org/wiki/Using_OS_Shapefiles
>>
>> and the converted data looks good to me compared to my previous 
>> surveys but comes out different to the tiles, so I'm thinking that the 
>> tiles are
> wrong.
>>
>> Kevin
>>
>>
>>
>>
>> On 30 May 2010 09:08, Tim François <sk1ppy14 at yahoo.co.uk 
>> <mailto:sk1ppy14 at yahoo.co.uk>> wrote:
>>
>>     > On a side note, has anybody noticed a consistent tendency
>>     > for existing
>>     > independently surveyed roads to be offset northwards (by
>>     > around 5-10
>>     > metres) from the OS data (vectormap and streetview)? I've
>>     > seen various
>>     > cases of existing roads being edited to be consistent with
>>     > OS data,
>> 
>    > but I'm not convinced this is a good idea since the problem
>>     > seems to
>>     > be consistent in one direction.
>>
>>     Glad I'm not the only one. Here in the SW I see the same offsets,
>>     although I find the VectorDistrict data to be more like the GPS
>>     surveyed data. This means that the StreetView tiles do not match
>>     up with the VectorDistrict either: I've been importing some rivers
>>     and reservoirs from the VectorDistrict data (namely the River Chew
>>     and Chew Valley Lake) and I've found that the polygons seem to be
>>     shifted compared to the equivalent positions in StreetView by
>>     about 10 metres.
>>
>> 
>    I guess this is an expected artifact of the reprojection methods?
>>
>>     Tim
>>
>>
>>
>>
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