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

[Smart, Gary]

Its true that the grid is only slightly divergent from true north over
the width of the UK.  However, we are stitching all the map tiles
together and need North to always be straight up the image.

I shall experiment with the gdal warp params suggest by Frank and see
how bad the warp is.

> -----Original Message-----
> From: Peter J Halls [mailto:P.Halls at york.ac.uk]
> Sent: 28 January 2010 07:25
> To: Smart, Gary
> Cc: gdal-dev at lists.osgeo.org
> Subject: Re: [gdal-dev] Re-aligning maps from grid north to true north
> 
> 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,
> >
> 
> --
>
------------------------------------------------------------------------
--
> ------
> 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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> ------