{"id":5969,"date":"2018-08-13T04:17:45","date_gmt":"2018-08-13T02:17:45","guid":{"rendered":"http:\/\/www.sigterritoires.fr\/?p=5969"},"modified":"2018-10-17T12:07:06","modified_gmt":"2018-10-17T10:07:06","slug":"how-to-manage-the-nw-africa-grid-projection-with-arcgis-and-qgis","status":"publish","type":"post","link":"https:\/\/www.sigterritoires.fr\/index.php\/en\/how-to-manage-the-nw-africa-grid-projection-with-arcgis-and-qgis\/","title":{"rendered":"How to manage the NW Africa Grid projection with ArcGis and QGis"},"content":{"rendered":"

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.
\nNow we will discuss the system used for the North African theater.
\nThe only ones detailed maps available for the Maghreb countries (Morocco, Algeria, Tunisia) were the French pre- war maps (1920-1936).<\/p>\n

\"\"<\/p>\n

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.<\/p>\n

\"\"<\/p>\n

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.<\/p>\n

\"\"<\/p>\n

NW Africa Grid projection<\/strong> management<\/strong> in<\/strong> ArcGIS<\/strong><\/p>\n

You will not find this projection within the available as standard with ArcGis.
\nOnce 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. \u00a0Indeed, 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.<\/p>\n

The parameters of the NW Africa Grid projection are as follows :<\/p>\n

False_Easting<\/strong> 1,000,000.0<\/strong>
\nFalse_Northing<\/strong> 500,000.0<\/strong>
\nCentral_Meridian<\/strong> 0 \u00b0 E<\/strong>
\nStandard_Parallel_1 34 \u00b0 N<\/strong>
\nScale_Factor<\/strong> 0.9994<\/strong><\/p>\n

As for the geodesic system, the parameters are those of the present data in the ArcGis list:<\/p>\n

Name: GCS_NTF<\/strong>
\nAngular Unit:<\/strong> Degrees<\/strong>
\nPrime Meridian: Greenwich<\/strong>
\nDatum: Clarke_1880_IGN1<\/strong>
\nSpheroid: Bessel 1841<\/strong>
\nSemi major<\/strong> Axis: 6378249.2<\/strong>
\nSemi minor<\/strong> Axis: 6356515.0<\/strong>
\nInverse Flattening: 293.4660212936265<\/strong><\/p>\n

Click on the following link to download a file . prj with all the necessary definitions.<\/p>\n

\"\"<\/p>\n

NW Africa Grid<\/a><\/p>\n

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:<\/p>\n

\"\"<\/p>\n

Setting<\/strong> up transformations<\/strong> in<\/strong> ArcMap<\/strong><\/p>\n

The transformation of the GCS_NTF datum is done through a standard transformation available in ArcMap.<\/p>\n

Click on the transformations button and select the desired transformation.<\/p>\n

NW Africa Grid projection<\/strong> management<\/strong> in<\/strong> QGis<\/strong><\/p>\n

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<\/a> page, you won\u2019t have to do anything else.<\/p>\n

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

\"\"<\/p>\n

NW Africa Grid projection war generated from the Arcgis file .aux<\/p>\n

You could be satisfied with such a result, but in a few days, the title SRC<\/a><\/em> Generated<\/em> followed by a set of parameters won\u2019t mean much to you.<\/p>\n

Therefore, you should save the parameters with a more explicit name:<\/p>\n