[gdal-dev] Re-aligning maps from grid north to true north

[Peter J Halls]

Gary,

    what do you mean by 'true north'?

    OSGB36 is a Transverse Mercator projection: it is, therefore, automatically 
aligned with a (reference) line of longitude, its central meridian.  It is not 
aligned to magnetic north: those little diagrams on the maps show the offset to 
magnetic north, which is constantly changing; assuming true north to be 90 
degrees, where all the lines of longitude meet, then there is no need for a 
change in alignment, as the northings of the OSGB36 grid are aligned with their 
central meridian.  There is, indeed, a slight error as one moves to the east or 
west of the central meridian, but this is so small within the latitudes of Great 
Britain that any correction would be too small to notice: this is a design 
feature of OSGB36.  It is this error that requires Ireland to have its own 
projection: it is sufficiently far west that the error is noticeable there.

    Everything that has been said on this list is correct for reprojection: my 
query is whether you actually need to re-project these data.

Best wishes,

Peter

Frank Warmerdam wrote:
> On Thu, Jan 28, 2010 at 2:01 AM, Smart, Gary <Gary.Smart at goodrich.com> wrote:
>> We have tif maps which are OSGB grid aligned, but we need them aligned to
>> true-north.  I presume this could be achieved by gdalwarp.  Can anyone give
>> me a clue how to do this?  Am I right in thinking that simply setting the
>> correct s_srs and t_srs would achieve everything I need?  If so – what part
>> of the WKT would relate to Grid vs True?
> 
> Gary,
> 
> It sounds like you want to reproject into a coordinate
> system that is aligned with the geographic lines of
> longitude.  The most obvious candidate coordinate
> system is geographic coordinates themselves.  If
> your existing grids are properly recognised as OSGB
> British National Grid, then it would be sufficient to do
> something like:
> 
>   gdalwarp -t_srs WGS84 in.tif out.tif
> 
> If your input files do not have the coordinate system
> set properly, but *do* have georeferencing set in
> terms of british national grid meters, then you could
> force gdalwarp to understand the input coordinate system
> like this:
> 
>   gdalwarp -s_srs EPSG:27700 -t_srs WGS84 in.tif out.tif
> 
> There are some other projected coordinate systems
> which are also aligned with the lines of longitude,
> including Mercator.
> 
> Best regards,
> 

-- 
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Peter J Halls, GIS Advisor, University of York
Telephone: 01904 433806     Fax: 01904 433740
Snail mail: Computing Service, University of York, Heslington, York YO10 5DD
This message has the status of a private and personal communication
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