### How to manage the NW Africa Grid projection with ArcGis and QGis

During World War I (1939-45) the British cartographic service was conveyed to produce a heaps of maps for all the conflict zones. In previous articles we have discussed projections used for the European theaters of war.
Now we will discuss the system used for the North African theater.
The only ones detailed maps available for the Maghreb countries (Morocco, Algeria, Tunisia) were the French pre- war maps (1920-1936).

The problem of these maps was basically the mesh in grades relative to the meridian of Paris, very French feature. The simplest solution found was to overprint on the original maps a mesh in meters, calculated relative to the Greenwich meridian.

Therefore we find in these maps an original mesh in black colour (not to be used), and the mesh used during the conflict 1939-45 in purple. The latter is the one used for georeferencing these maps.

NW Africa Grid projection management in ArcGIS

You will not find this projection within the available as standard with ArcGis.
Once the projection is created, it will not be necessary to enter the transformation methods towards the usual systems (WGS84, NTF, and RGF93) for other projections of the conflict.  Indeed, the maps of the Maghreb of that period are based on the Clarke 1880 ellipsoid, as well as the metropole projections (Lambert I, II, etc.). Therefore we will be able to use the NTF processing for these maps.

The parameters of the NW Africa Grid projection are as follows :

False_Easting 1,000,000.0
False_Northing 500,000.0
Central_Meridian 0 ° E
Standard_Parallel_1 34 ° N
Scale_Factor 0.9994

As for the geodesic system, the parameters are those of the present data in the ArcGis list:

Name: GCS_NTF
Angular Unit: Degrees
Prime Meridian: Greenwich
Datum: Clarke_1880_IGN1
Spheroid: Bessel 1841
Semi major Axis: 6378249.2
Semi minor Axis: 6356515.0
Inverse Flattening: 293.4660212936265

NW Africa Grid

To assign this projection to the data block or to a GIS layer, open the window for defining coordinate systems and click on the import button:

Setting up transformations in ArcMap

The transformation of the GCS_NTF datum is done through a standard transformation available in ArcMap.

Click on the transformations button and select the desired transformation.

NW Africa Grid projection management in QGis

Technically with QGis it’s much more simple than with ArcGis. If you download one of the maps with the NW Africa Grid available in the Maps page, you won’t have to do anything else.

QGis reads the file .aux of ArcGIS present in the download and automatically creates the corresponding custom projection. If you open the menu Preferences -> Custom projection you will see:

NW Africa Grid projection war generated from the Arcgis file .aux

You could be satisfied with such a result, but in a few days, the title SRC Generated followed by a set of parameters won’t mean much to you.

Therefore, you should save the parameters with a more explicit name:

• click the +button (addition of a projection)
• in Name return NW Africa Grid / NTF
• in Settings copy and paste those of the created SRC
• + proj = lcc + lat_1 = 34 + lat_0 = 34 + lon_0 = 0 + k_0 = 0.9994 + x_0 = 1000000 + y_0 = 500000 + a = 6378249.2 + b = 6356515 + units = m + no_defs
• click OK

In the list of available screenings you can enter NW Africa Grid in the filter and find simply the location of this projection.

ATTENTION! The downloaded files have a georeferencing file (World) with the extension. jgwx ( generated by ArcGis). QGis, for the time being, does not recognize this extension. You must modify it to jgw , before loading an image, so that the georeferencing is taken into account .

Si cet article vous a intéressé et que vous pensez qu'il pourrait bénéficier à d'autres personnes, n'hésitez pas à le partager sur vos réseaux sociaux en utilisant les boutons ci-dessous. Votre partage est apprécié !