French trench map location

[LDT006]

I have the following French coordinates: East 172.2 North 387.6 and East 187.2 North 387.6

These should be in the Cambrai area but I can't find a French map of that area,
Anyone who can find these locations?

Thanks.

[busterfield]

You weren't the only one looking. This may be of help.

 

[Howard]

This is going to sound convoluted but that is a result of the fact that we do not have a French map with Lambert co-ordinates for that area on TrenchMapper.

Lambert co-ordinates are in metric so 172.2 is 172.2 kilometres east of the origin etc. 170 is shown on the map so the extra 0.2 is just 20% of the way to 173. Note the Lambert grid (red) is angled to the black British grid.

The nearest we have is m_5_005348, you can get that by right clicking, choose Map ID Jump and enter m_5_005348. That has its margins cropped off but here is the NE corner. Not quite there so I extended the grid a bit then plotted 172.2, 387.6.

Havind done that, I drew a yards based triangle in blue based from Bourlon Church. With Pythagoras, that came out as a distance of 3,028 yards bearing 40.3 degrees. You can use that result that in TrenchMapper with the bearing tool from Bourlon Church to show your first point. Just repeat that process for your other point in any graphics software.

Howard

[Howard]

Alternatively, noting that in 172.2 , 387.6 and 187.2, 387.6  the Northings of the second point are that same at 387.6, project a point from the first point at the same bearing that the red Lambert grid makes, 86.3 degrees, i.e. staying at Northings 172.2. As 187.2 - 172.2=15 or 15,000 metres that is 16,404 yards, so using the TrenchMapper bearing tool, go from the first point 16,404 yards at 86.3 degrees to get the second point then add a marker.

Naturally this is approximate! If we find a Lambert gridded map of the area we will add it to TrenchMapper.

Howard

[AOK4]

Have you tried geoportail.gouv.fr? There you can select the 1950s maps with Lambert grids (I assume).

I noticed there are free online tools that convert coordinates as well

Jan

Edited 15 August , 2023 by AOK4

[LDT006]

Howard,

thanks very much for your help, I don't understand your triangulation and how to do it in Trenchmapper.

Those map references should be for the Rieux communal cemetery, only one can be correct. The 172.2 387.6 that you plotted is obviously wrong.
I found a map (number 13 NE) but without the Lambert coordinates

So I used Google earth and added two markers on the 386 line from the map that you posted.
Then drew a line through these points, added a second line from the cemetery square to the first line and placed a third marker there.
Using the measure tool and some calculation confirms that the second reference (187.2 387.6) is the correct one.

I know that this isn't perfect but good enough for me.

 

[brianmorris547]

I had a look at the Div HQ papers of those who were at Le Cateau in August 1914 and all I could find was a 1:80 000 map of Cambrai in two parts in the WD of 4 Div HQ GS. I don't think it is of much use though. There are some Lat and Long co ordinates in the top and bottom corners. It seems to be a British map. Attached is the half which shows Cambrai.

TNA WO 95/1439

Brian

[LDT006]

On 15/08/2023 at 20:14, AOK4 said:

Have you tried geoportail.gouv.fr? There you can select the 1950s maps with Lambert grids (I assume).

I noticed there are free online tools that convert coordinates as well

Jan,
I found those 1950 maps but it's only one for the whole of France and no Lambert grids.
It looks similar to what we have in Belgium. We need the separate maps by number, I have some for Belgium, scale 1/25.000 from the Military geographic institute. Ex 28/3-4 Geluveld and Moorsele. These have Lambert markings but I can't find these online or am I missing something.
I also found an online tool (cconvert) but it's way to complicated for me. If there is a simple one them maybe @WhiteStarLine and/or @Howard could implement it in Trenchmapper.

On 17/08/2023 at 19:25, brianmorris547 said:

all I could find was a 1:80 000 map of Cambrai

Brian,
thanks for looking.
There are some more detailled maps of Cambrai (and other French maps) here:
https://digital.ncdcr.gov/documents?filter_24=World%20War%20I%20Maps&applyState=true

Luc.

[brianmorris547]

Might this old thread help https://www.greatwarforum.org/topic/248483-maps-of-the-french-artillery-survey-group-1917-ca-1919/#comment-2528489

Brian

[Howard]

Sadly the issue of Great War French map projection is complex, confusing and full of errors. Col. Jack in his Report on Survey on the Western Front (available in the Help->Knowledge Centre of TrenchMapper) was very rude about the French cartographers, most of the issues are shown in that report. They had been 100 years ahead of the British in cartography, notably from the efforts of the amazing Cassini family but by the Great War, their mapping output was woeful.

There were good bits, notably the Plans Directeur. The Lambert projection used the same mathematics as the one still used in France but the Figure of the Earth used was not, i.e. the size of the earth, radius, flattening etc. Lambert splits France in three regions, North, Central and South but the Great War maps used a figure of the earth and zoning invented by the British called Nord de Guerre (still used in the 2nd war). This came from the British need to remap the battle areas and to correct or ignore the numerous errors in the French cartographic tables and geodetic survey data. Naturally this annoyed the French but it means that one cannot use modern (on-line 3 zone) Lambert converters as they use a modern figure of the earth. The more modern term is CRS.

Taken that the Plans Directeur series were good maps, the best the French had produced, have a look at this index map and note the sheets at different orientations to each other, a factor that makes a mathematical calculation of lat/lon coordinates from Lambert values difficult. It is hard enough doing the British conversion owing to numerous technical problems, but this and the lack of firm CRS details means the Lambert problem remains severe.

One would be forgiven for thinking that taking the lat/lon values in the corners of the 1:80,000 series (as shown in Brian’s post above) would give a useful result. They do not, the CRS or colloquially the “datum” is different. In an ideal world, a given position like 49° 14’ 23” N, 2° 34’ 12”E should fix one unambiguous point on earth. It doesn’t, very far from it, there are numerous points, sometimes as far a 1000 meters apart that share the same coordinate values, it all depends in where you measure the point from and the size of the earth assumed in that measurement. Quite apart from anything else, even if the CRS used in the Great War was the same as we use now (WGS84), the points will have moved about 2 metres NE from where they were owing to the movement of the European Tectonic plate.

Getting precise point coordinates is tricky and converting them from one CRS to another is fraught with uncertainties. I have covered a lot of these points in Help->Knowledge Centre on TrenchMapper. One could use a map and compass but this gives other problems as I outlined here.

That is why we still resort to drawing lines on maps.

Howard