Problems with matching layers with different projections

Question

I have two layers with the following projections: Monte Mario IT1 UTM*

Anyhow, I followed the instructions (open the project, set the projection, select on the fly projection, load the layers but no way, the two layers does not match each other.

I could only obtain the result of picture 1 where in red are the transport flows, in blue the transport network.

The crs of the two layers are: red--> +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl +towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68 +units=m +no_defs blue--> +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=500053 +y_0=-3999820 +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m +no_defs

If I set one of the layer with the same crs as the other, the two networks becomes very far from each others (see picture 2)

Which do you think might be the problem? Thank you, Selini

Answer 1

The first data layer is, as you said, using Italy Zone 1 / West zone based on Monte Mario geographic coordinate reference system. The second data layer's +proj definition looks like UTM* / UTMRER aka TM Emilia-Romagna (ID = 5659) in the EPSG Geodetic Parameter Dataset. Although I'm on the subcommittee that maintains the Dataset, I was on leave when this coordinate reference system was added, and I'm finding the emails about it a little confusing. Because it's listed as being on Monte Mario, and the offsets look like a GeoCRS / datum issue, try these on the 2nd data layer:

1. Change the +towgs84 parameter to use the same values as the first layer (Monte Mario to WGS84) - EDIT: I think this option will work; see below.
2. Omit it completely

to see if the data layers will overlay better.

I found an online translator. If you convert a WGS84 coordinate pair to UTMRER (aka UTM*) and to Monte Mario Zone 1 / West, the coordinates differ only by the differences in the false easting and false northing values so both must have undergone a WGS84-to-Monte Mario conversion.

Input:
longitude = 10.0E
latitude = 44.0N

Results: UTMRER
East: 580255.2 North: 872556.1

Monte Mario Italy 1
East: 1580202.2 North: 4872376.1
East: 1580202.2 - 580255.2 + 53.0 = 1000000.0
North: 4872376.1 - 872556.1 + 180.0 = 4000000.0