# Modified British System "South Italy Zone" grid - wide residual error georeferencing maps in ArcMap or QGIS

In accordance with the information of a previous thread on this platform, I am trying the georeference some war maps of Italy within the South Italy Zone grid of the Modified British System. I work with known points of a scanned map but, unfortunately, I get an average residual error of about 300-400 meters, both with QGIS and ArcMap, as well as using the 'coordinate translator' by Thierry Arsicaud’s website. I have never got a similar anomaly working with other map projections, and the results are always been acceptable.

However, They follow the projection parameters of the "South Italy Zone" grid:

``````Projection - Lambert Conical Orthomorphic
Ellipsoid: Bessel 1841
False Easting : 700000
False Northing : 600000
Central Meridian : 14.0°
Central Parallel : 39.5°
Scale Factor : 0.99906
``````

New Qgis projection parameters:

``````Projection - Lambert Conical Orthomorphic  →    +proj=lcc
Ellipsoid: Bessel 1841                     →    +ellps=bessel
False Easting : 700000                     →    +x_0=700000
False Northing : 600000                    →    +y_0=600000
Central Meridian : 14.0°                   →    +lon_0=14
Central Parallel : 39.5°                   →    +lat_0=39.5 +lat_1=39.5
Scale Factor : 0.99906                     →    +k_0=0.99906
(other proj.4 terms)     +units=m +no_defs
``````

the related Qgis proj.4 working definition:

``````+proj=lcc +lat_0=39.5 +lat_1=39.5 +lon_0=14 +k_0=0.99906 +x_0=700000 +y_0=600000 +ellps=bessel +units=m +no_defs
``````

According to an informative article about the same subject, I could add the "Italy South" grid projection in ArcMap too. It has the additional complication to need further computation for the transformation between different geodetic systems, rather than QGis.

They follow the parameters which I used in ArcGis for the geodetic system:

``````Name: Bessel 1841
Angular Unit: Degrees
Prime Meridian: Greenwich
Datum: D_Bessel_1841
Spheroid: Bessel 1841
Semimajor Axis: 6377397.155
Semiminor Axis: 6356078.963
Inverse Flattening: 299.1521535
``````

the parameters to set up the transformation from Bessel to WGS84:

``````dX = 682
dY = -203
dZ = 480
``````

and from WGS84 to Bessel:

``````dX = -682
dY = 203
dZ = -480
``````

Despite many attempts, I get the same wide residual error and I couldn't find a solution.

How can I calibrate the parameters in order to compensate the coordinate offset?

• This map custermen.com/Reference/Files/Map_ROMA_Sht-150.jpg states is uses the Rome meridian is 12.451972 at the bottom of the image. This map uses the same projection as your question.
– Mapperz
May 28, 2020 at 13:55
• I have a related problem with British WWI trench maps, which seem to use an early form of UTM. May 28, 2020 at 14:44
• Unfortunately, the kind of maps which I am trying to georeference use just the "Italy South" grid. They are not GSGS maps, they are Polish maps and there is not any other grid, or reference (lat/long) overlayed. May 28, 2020 at 15:49
• do you have a link to the maps?
– Mapperz
May 28, 2020 at 16:12
• Please decide whether you wish to ask about QGIS or ArcGIS Desktop in this particular question. You can always ask about the other in a separate similar question.
– PolyGeo
May 28, 2020 at 20:06