Hi Sarah There are two aspects involved in converting between BNG and LL.
If you are converting LL to BNG then first you need to convert from degrees to the mathematical position of BNG using the appropriate ellipsoid major and minor radii ( ie WGS84 or other ) and the BNG -2,49 origin, false origin and scale factor. Secondly you then need the table available from OS to move these theoretical positions to the "published" positions. Ordnance Survey provide this table on their website. It is a look up table about 24 MB in size. If you are converting BNG to lat long then you should reverse the above. ie convert the BNG "published position" to the mathematical position, then transform from BNG to LL. The published positions for BNG are based on older county series maps. The discrepencies between these and the mathematical positions cannot therefore be contained in formulae and the conversion must be done through the OS table. Typical shifts are about 100 metres. The typical difference I get just using MapInfo is about 6 metres. This implies that MapInfo have "built in" some sort of average adjustment of about 94 metres. Does anybody know if this is the case? If you use the table you will get agreement to 1 millimetre !! ( ie agreement to the figure on the OS Website that you currently use). However to be able to store this sort of accuracy remember to put a bounding rectangle around the UK so that the internal X and Y step in you MAP file is small. The OS Website contains very good information on the above. Hope this helps. I am building these adjustments into a Pocket PC application and a free MapBasic application at present. If anyone is interested email me and I will send you the free MBX when it is finished. Regards Bob MapsByDesign.co.uk Quoting Hooper Sarah <[EMAIL PROTECTED]>: > Hi, > > I need to change a set of Eastings/Northings (British National Grid) into > Lat/Longs, and although I know the actual process in MapInfo, the answers I > get differ to those I get through the OS Website (using Co-ord converter). > > Eg. > Using the co-ords below the different answers I get are as follows; > E529256, N178798 > > MapInfo - Lat 51.492847, Long -0.137903 > > OS site - Lat 51.49336039, Long -0.13948250 > > My query is, why the difference (am I using the wrong transformation?), and > if so can someone explain what I should be using instead (is there any > reading material out there so I can learn more). > > Thanks in advance, > Sarah > > > > ******************************************************************************** *** > The contents of the e-mail and any transmitted files are confidential and > intended solely for the use of the individual or entity to whom they are > addressed. Transport for London hereby exclude any warranty and any liability > as to the quality or accuracy of the contents of this email and any attached > transmitted files. If you are not the intended recipient be advised that you > have received this email in error and that any use, dissemination, > forwarding, printing or copying of this email is strictly prohibited. > > If you have received this email in error please notify > [EMAIL PROTECTED] > > This footnote also confirms that this email message has been swept for the > presence of computer viruses. > ******************************************************************************** *** > > > --------------------------------------------------------------------- > List hosting provided by Directions Magazine | www.directionsmag.com | > To unsubscribe, e-mail: [EMAIL PROTECTED] > For additional commands, e-mail: [EMAIL PROTECTED] > Message number: 10219 > > --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 10269
