[GRASSLIST:8229] Re: Gauss-Boaga location and wgs84 proj parameter

[Clifford J Mugnier]

Luca,

None of those are correct.  What you have listed are the datum shift
parameters between European Datum 1950 and the WGS 84 datum.

Gauss-Boaga is on Rome 1940 datum (Castello Monte Mario).  For a background
on the history and origins of Italian Geodesy, Grid Systems, etc., check
out my column on the Italian Republic in Photogrammetric Engineering and
Remote Sensing, Grids and Datums column at:

www.ASPRS.org/resources.GRIDS

(August 2005)

I suggest that you also look up my column on the "Basics of Datums" at that
same url and do some reading on the difference between a Grid (Gauss-Boaga
East and West zones) and a Datum (WGS84, Rome 1940, ED50, etc.).

Your English is fine.  It is substantially better than my Italian!  :-)

Clifford J. Mugnier
Chief of Geodesy and
Associate Director,
CENTER FOR GEOINFORMATICS
Department of Civil Engineering
CEBA 3223A
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic]
Voice and Facsimile:  (225) 578-4474 [Research]
======================================================
http://www.asprs.org/resources/GRIDS/
http://www.cee.lsu.edu/facultyStaff/mugnier/index.html
======================================================




Im going to create a Gauss-Boaga location where to load some georeferenced
maps. This is what I have done:

1.

Please specify the coordinate system for location <tempo>
A   x,y
B   Latitude-Longitude
C   UTM
D   Other Projection
RETURN to cancel

> D -> I'll create the location with the Other Projection procedure.

2.

Please specify projection name
Enter 'list' for the list of available projections
Hit RETURN to cancel request
>tmerc  -> i think is the right projection

3.

Please specify datum name
Enter 'list' for the list of available datums
or 'custom' if you wish to enter custom parameters
Hit RETURN to cancel request
>rome40 -> it is right, isn't?

4. ------here is the problem----------

Number  Details
---
1       Used in Italy (Peninsular Part)
        (PROJ.4 Params towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68)
        Accuracy 3-4m
---
2       Used in Italy (Sardinia)
        (PROJ.4 Params
towgs84=-168.6,-34.0,38.6,-0.374,-0.679,-1.379,-9.48)
        Accuracy 3-4m
---
3       Used in Italy (Sicily)
        (PROJ.4 Params towgs84=-50.2,-50.4,84.8,-0.690,-2.012,0.459,-28.08)
        Accuracy 3-4m
---
4       Used in Default rome40 region
        (PROJ.4 Params towgs84=-225.000,-65.000,9.000)
        Default 3-Parameter Transformation
---

What is the right number? i would think that is the 1st (couse im in the
north of Italy) or the 4th
How can i know which towgs84 parameter have i to use?
and.. if the right parameter is the first, why is there the 4th parameter,
that is so strange?


sorry for my english.... thanks

Luca P.