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 <[email protected]> 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
--------------------------------------------------------------------------------
_______________________________________________
gdal-dev mailing list
[email protected]
http://lists.osgeo.org/mailman/listinfo/gdal-dev
