Magnetic North or True North?

[Alan24]

I know this sounds like a schoolboy question but just want to be clear in my own mind...

 

The grid system used on the trench maps (as seen on websites like NLS and MacMaster) is this based on True North or Magnetic (Compass) North.

 

I notice that the trench maps on NLS are slightly rotated to align with Google Earth.

 

Am I correct in thinking that the difference between the two is less today than 100 years ago? 

 

Regards

 

Alan.

[charlie2]

On a map it is grid north, grid north needs to be aligned to true north with the help of a compass allowing for the magnetic deviation which changes every year. When converting bearings - grid to mag - add, mag to grid get rid. The magnetic variation is sometimes found in the map margin, see here http://digitalarchive.mcmaster.ca/islandora/object/macrepo%3A4259/-/collection

 

Hope this helps

Charlie

[RobertBr]

A query I had answered a while ago

 

Magnetic Declination

[Alan24]

Thanks Guys,

 

That gives me a clue. I'll try and put it into practice next week.

 

Regards

 

Alan.

 

 

[Lawryleslie]

There are three norths for map and navigational purposes : True North, Grid North and Magnetic North. To explain the difference between Grid North and Magnetic North is straight forward in that magnetic north is based on the earths magnetic field that is centred on a point in northern Canada where all magnetic compasses point to, whereas Grid North are the vertical lines on maps pointing towards the Northwards but not the North Pole. The difference between Magnetic North and Grid North give us the the magnetic deviation and in U.K. it is currently about 2 degrees taken away when converting from magnetic to Grid North. True North is the geographical North Pole depicted by the curved lines of  Longitude and is very close to Grid North in U.K. to be negligible. If we used True North on maps the vertical lines would be slightly curved particularly on large scale maps and this would make compass work inaccurate.

 

Going back to Magnetic North and why this changes annually, imagine a great lump of iron under the earths surface in northern Canada that is free floating. As the earth occilates about it’s  vertical axis this lump of iron, being of greater mass than the earth, moves in relation to its geographical position annually due to tectonic plate push and other forces and is known scientifically as the Polar Shift Theory. This movement is stated in +/- minutes of angle annually in the maps margine. 

Edited 28 May , 2018 by Lawryleslie

[NigelS]

Here's the Ordnance Survey's explanation Click which includes a link to the British Geological Survey's (BGS) Magnetic angle calculator to allow determination -  there are subtle differences from place to place (& therefore from OS sheet to sheet) - of  the current variation for locations in the UK. 

 

NigelS

[Howard]

You have to take care to get this right. The explanations above are all right, there are three norths but sadly some trench maps give only two and sometimes do not state explicitly which is which. I found these marginal notes, one that appears to show true North with an "N" on the arrow showing the large variation with magnetic north. The N arrow is not parallel to the grid. On the other one, magnetic north is shown explicitly in relation to grid north. Sadly some cartographers then and now miss off vital information, it seems to be a feature of their way of working. The difference between true North and grid North will vary depending on the distance East or West of the central meridian.

 

100 years ago the magnetic variation was so large that getting it wrong could have been disastrous. An error of half a degree may be OK if the purpose of the measurement was to identify a distant church spire but would be a large error for artillery.

 

Howard

 

[MBrockway]

One should add that Grid North is only consistent for maps using the same grid. 

 

Current UK Ordnance Survey maps use the UK OS National Grid, but British Great War trench maps did not have the benefit of an existing grid and had to rely on the surveys of the French and the Belgian nation mapping agencies to derive a model of lat and long before even constructing a grid.  This diagram shows some of the discrepancies:

 

 

 

 

It goes without saying that grid north for the OSGB National Grid is different from grid north of Great War Western Front trench maps.  Our current GB National Grid and its immediate predecessors only go as far back as 1927 anyway!

 

The problems of survey and geodetics gave serious headaches right through the first three years of the war.  A full description of the complexities would require a very long post, but they are described in detail in Colonel Jack's Report on Survey on the Western Front 1914-1918 and are a must-read for anyone with a genuine interest in Great War mapping.

 

The rotation of the trench maps versus a Google Earth map background is due to geodetic issues not  magnetic variation.  Again reading Jack's report will explain this.

 

 

As regards the True/Grid/Magnetic diagrams in map margins, this section from Jack's report is informative ...

 

 

Bearings relative to True North are of course relevant where you are obliged to work with Lat and Long instead of having the benefit of a "rectangular" series of maps with a grid reference system.

 

Since UK military navigation teaching was probably aimed at use across the Empire, one can see perhaps why True North was still being taught well into the war.  I'm unsure if theatres outwith the Western Front had 'gridded' mapping available or had to rely on Lat and Long as per naval practice.  IIRC there was a 'grid' for Gallipoli.

 

Mark

 

[Howard]

On 26/05/2018 at 17:14, Alan24 said:

I notice that the trench maps on NLS are slightly rotated to align with Google Earth.

 

All maps are a distortion because they are flat and the earth is not. When a map re-projected, this distortion sometimes becomes more obvious. Grid lines that are "straight" are often curves but look straight on a small map.

 

Google Earth uses a simple cylindrical projection and WGS84 as the datum. Google Maps, Bing, Microsoft Virtual Earth and other web maps use a Mercator projection but also using WGS83 as the datum. Most British trench maps used the Bonne projection and a datum that was not explicitly named but was probably based the old French Nord de Guerre. This map projection and datum was inherited from Belgian maps and their grid system was extended across France as an emergency operation in 1914, hence many maps of France have a letter added to the sheet numbers, e.g. 36C, as the Belgian system was never intended to extend into France. The result was a dog's breakfast as far as map are concerned.

 

No one would have chosen the Bonne projection for a map used for artillery because the projection does not preserve bearings over distance. The French used the Lambert projection which does, so plans were made for the British mapping to change to the Lambert system late in the war, a few British maps exist with just the Lambert grid or both.

 

The upshot of all this is when a map is re-projected onto the Google Earth cylindrical projection is looks odd. As Mark's post shows, the detailed situation was made worse by the less than acceptable level of accuracy in the French and Belgian geodetic survey data. The confusion over the actual datum used for both British Bonne maps and the French Lambert maps probably comes from these errors, e.g. attempts to place Great War maps on Google Earth using fundamental values (as opposed to fixed points like road junctions) often fail, the re-projection required not being based on accurate or even known values. One can make a good stab at it using fixed points but sometimes is just looks "odd".

 

Howard

Edited 29 May , 2018 by Howard
Typo

[MBrockway]

I gave up trying to explain the geodetic issues in simple terms in a short post, but Howard has covered it well.

 

Pals might also find interesting/informative this paragraph on the Great War grid systems from Winterbottom's Preliminary Report on Survey from 1918 ...

 

 ... and Winterbottom's points re how specialised the work of triangulation and fundamental surveying really is are also noteworthy ...

 

 

 

 

[Howard]

Col. Jack in his Report on Survey on the Western Front 1914-1918 was also quite rude about the French and Belgian mapping. Although couched in official language, it is clear that he and his Royal Engineer colleagues thought that especially the French had bungled much of their geodetic work and left undone all sorts of vital work. That is surprising as the French were at one point years ahead of the British as regards geodetics. The Ordnance Survey was modelled on much of what the French had done in establishing the Figure of the Earth and related geodetic surveying. The French also had their superb Plans Directeur (Lambert) for much of the British sector of the Western Front yet no-one thought of telling us so we were lumbered with geodetic tables full of errors, the wholly ghastly 1:80,000 maps from the 1890s and Bonne maps redrawn from these and hence were quite inaccurate. British re-survey started very early as a result. Interestingly, the Germans were in a worse state and continued to use re-drawn 1:80,000 maps until the end.

 

Howard

 

Report on Survey on the Western Front 1914-1918 on my 1/Middlesex website

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