[OSRS-PROJ] Projection of historic maps WW II - Italy

[Markus Neteler]

Dear List members,

I would appreciate assistance for the following problem:

Currently I try to define a map projection for a map from 1945,
Northern Italy. The map was made by the US Army Engr. Top. Co.
in Lambert Conic Orthomorphic. It is based on a map from 1931 
done by Istituto Geografico Militare and was enhanced by aerial
photo surveys in 1944.

Given on the (scanned) paper map is:
- prime meridian Monte Mario (old!): 12d27'7.1E (in PROJ: 12d27'8.4E)
- "spheroid":     Bessel
- Origin:         14E, 45:54N
- False Easing:   800000 meters
- False Northing: 601000 meters
- The map provides the LCC grid and coordinates as well as
  Lat/Long corners in reference to Monte Mario 12d27'7.1E (given as
  "West from Rome" and Equator.

This projection I am defining in GRASS GIS which is based on the latest
PROJ from CVS. The parameters in GRASS style are (PROJ_INFO file)

name: Lambert Conformal Conic
towgs84: -104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68
proj: lcc
ellps: bessel
a: 6377397.1550000003
es: 0.0066743722
f: 299.1528128000
lat_1: 45.9
lon_0: 14.0000000000
x_0: 800000.0000000000
y_0: 601000.0000000000

The towgs84 parameter will be certainly wrong.

Using these values I achieve as accuracy a shift 70m eastwards and 
170m northwards. For verification GIS data in Gauss-Boaga projection
from different providers were using (vector street map and orthophotos).
These maps I have reprojected into above LCC projection and compared to
the scanned and geocoded map from 1945:

For illustration there is a screenshot of a zoomed area:
http://mpa.itc.it/markus/tmp/lcc_1945_ortho2000.png
(1MB)
http://mpa.itc.it/markus/tmp/lcc_1945_ortho2000_small.png
(500kb)

To improve the accuracy, I am seeking assistance to find out:
- which Bessel definition should be used (is above right for these 1945 maps?)
- which datum was used in 1945 (Italian or US or ...)
- how to define a different prime meridian: 12d27'7.1E instead of 12d27'8.4E
  (can I define a custom prime meridian?)

Thanks in advance

 Markus Neteler
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