RobertBr Posted 10 December , 2008 Share Posted 10 December , 2008 Can anyone confirm the Magnetic Declination in 1918 as used on Western Front trench maps. On my copy of 51B NW1 dated March 1918 the Magnetic Declination is virtually unreadable, it appears to be 1x deg 1y min West of Grid North where x and y could be '2' or '5' or vice versa! I have assumed Grid North to be True North. I am developing a spreadsheet that will calculate Range and Bearing of one set of Trench Map co-ordinates from another and then work out Lat Lon. Bob Link to comment Share on other sites More sharing options...
joseph Posted 10 December , 2008 Share Posted 10 December , 2008 Bob 51b NW.1 edition 11, July 1918, 11 degrees 38 minutes West of grid North. Regards Charles Link to comment Share on other sites More sharing options...
14kvn Posted 10 December , 2008 Share Posted 10 December , 2008 QUOTE (Bob B @ Dec 10 2008, 12:47 PM) <{POST_SNAPBACK}> Can anyone confirm the Magnetic Declination in 1918 as used on Western Front trench maps. On my copy of 51B NW1 dated March 1918 the Magnetic Declination is virtually unreadable, it appears to be 1x deg 1y min West of Grid North where x and y could be '2' or '5' or vice versa! I have assumed Grid North to be True North. I am developing a spreadsheet that will calculate Range and Bearing of one set of Trench Map co-ordinates from another and then work out Lat Lon. Bob Bob Sorry but you have assumed incorrectly. There are Three North's on a UK Map and now I'm assuming that if the Trench maps you refer to are of British origin they would be similar . Grid North: the direction of a grid line which is parallel to the central meridian on the National Grid. True North: the direction of a meridian of longitude which converges on the North Pole. Magnetic North: the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate and currently is west of Grid North in Great Britain. I think you would have to Contact The Ordnance Survey to get relevant values. Regards Kevin Link to comment Share on other sites More sharing options...
RobertBr Posted 12 December , 2008 Author Share Posted 12 December , 2008 Charles Thanks for the info. I have blown up a section of my map which I now read as 13 (or 15) deg 12 min. The map is clearly hand marked to say that the dispositions were correct on 28th March 1918. So I assumed it is a 1918 map, but could it be 1916? I think this is unlikely. Magnetic Declination is currently changing at about 30min over 3 years (ref Ordnance Survey). Its rate of change, ~10min per year, may have been different in 1918 but I suspect not by that much. This leaves me with a conundrum of how to reconcile your July 1918 figure with mine! Kevin It was a little 'tongue in cheek' when I wrote that I assumed Grid North was True North! I agree with your statement with respect to UK maps. However would the central Meridian for UK maps have been used for Western Front Maps? Bob Link to comment Share on other sites More sharing options...
joseph Posted 12 December , 2008 Share Posted 12 December , 2008 Bob, I will see if I can find any more "Magnetic Declination" for 1918 it may give you a better picture. Regards Charles Link to comment Share on other sites More sharing options...
Smithmaps Posted 13 December , 2008 Share Posted 13 December , 2008 QUOTE (Bob B @ Dec 10 2008, 12:47 PM) <{POST_SNAPBACK}> I am developing a spreadsheet that will calculate Range and Bearing of one set of Trench Map co-ordinates from another and then work out Lat Lon. Bob Bob you have set yourself a tall order! Alternatively, you could just ask anybody with LinesMan to give you the Lat Long you require Or even post it here, as I can convert for you. Guy Link to comment Share on other sites More sharing options...
joseph Posted 13 December , 2008 Share Posted 13 December , 2008 Bob, Spanner in the works, Jan 1918 12* 12"...... looking at the world "Magnetic Declination" map it looks like 5* per anum, maybe one of our French members can find the annual declination for you, they are readily available for ordnance survey in UK. Regards Charles Link to comment Share on other sites More sharing options...
RobertBr Posted 13 December , 2008 Author Share Posted 13 December , 2008 I have just found a chart on the British Geographic Societies website that shows a rate of change of 9 to 10 minutes per year during the war. My spreadsheet and macros work quiet well allowing me to calculate a Lat/Lon from Trench Map coordinates. I am hoping to refine it a little to compensate for the differences between today's and WW1 Magnetic Declination. Bob Link to comment Share on other sites More sharing options...
Guest geoff501 Posted 13 December , 2008 Share Posted 13 December , 2008 I guess the rate varies with time (since the magnetic pole rotates about the true north pole - if memory serves me well). It will also depend on both latitude and longitude, so you need local values. (I think ) Link to comment Share on other sites More sharing options...
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