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    <p>This type of conversion is called a "geodetic transformation". OS and UTM are both "<a href="http://www.nearby.org.uk/tests/GeoTools.html" rel="nofollow">transverse mercator</a>" projections, wherein the ellipsoid of the earth is unwrapped into a cylinder, which is then unrolled into a flat sheet and sub-divided into grid sections. OS coordinates are specific to regions (eg: OSGB for Great Britain), whereas UTM is a "universal" system and specifies a system of grids for the whole earth. Regional grids are used in order to reduce the side-effects of distortion introduced by the mercator projection. It follows that converting between such systems is possible, but can also be quite complex depending in the accuracy desired.</p> <p>It seems there are only indirect methods, as you have already referred to, the most common being to convert from OSGB36 to WGS84 (lat/long) and then to UTM. </p> <p>Here are some resources which might be helpful:</p> <ol> <li>Convert WGS84 lat/long to UTM: <a href="http://www.uwgb.edu/dutchs/usefuldata/utmformulas.htm" rel="nofollow">http://www.uwgb.edu/dutchs/usefuldata/utmformulas.htm</a>. Note the inclusion of specific parameters for each region. For example, if you were converting coordinates for Britain, the parameters for "Airy 1830" would be used. (also links to a spreadsheet and webpage with conversions).</li> <li>Similar information as above on <a href="http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system" rel="nofollow">Wikipedia</a>.</li> <li>JavaScript to convert OSGB36 to WGS84 (7 metre accuracy): <a href="http://www.nearby.org.uk/tests/GeoTools.html" rel="nofollow">http://www.nearby.org.uk/tests/GeoTools.html</a></li> <li>A more accurate JavaScript conversion using a Helmert transformation (5 metre accuracy): <a href="http://www.movable-type.co.uk/scripts/latlong-convert-coords.html" rel="nofollow">http://www.movable-type.co.uk/scripts/latlong-convert-coords.html</a> and <a href="http://www.movable-type.co.uk/scripts/latlong-gridref.html" rel="nofollow">http://www.movable-type.co.uk/scripts/latlong-gridref.html</a></li> <li>Comprehensive coverage of the OSGB36 coordinate system, including transformations to and from other coordinate systems: <a href="http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf" rel="nofollow">http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf</a></li> <li>Miscellaneous links and resources: <a href="http://www.ordnancesurvey.co.uk/oswebsite/gps/information/resourceinfolinks/gpslinks.html" rel="nofollow">http://www.ordnancesurvey.co.uk/oswebsite/gps/information/resourceinfolinks/gpslinks.html</a></li> </ol> <p>As for accuracy, it is summed up in this <a href="http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/Geomatics_world.pdf" rel="nofollow">excerpt from ordnancesurvey.co.uk</a>:</p> <blockquote> <p>... OSGB36 contains randomly variable scale errors, mainly due to it being computed in blocks and the fact that scale and azimuth were controlled entirely by the 11 stations from the Principle Triangulation. These scale variations mean that OSGB36 can be described as inhomogeneous ... The inhomogenity of OSGB36 does not affect its adequacy as a mapping datum but it does make a simple transformation between ETRS89 and OSGB36 too inaccurate for national use. For example, the accuracy of a national 7 parameter (3 shifts, 3 rotations and a scale change) transformation is approximately 5 metres</p> </blockquote> <p>Here is a <a href="http://docs.codehaus.org/display/GEOTOOLS/ArcInfo+ASCII+Grid+format#ASCIIGrid" rel="nofollow">link to more comprehensive information regarding the ARC/INFO file format</a>.</p>
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