My problem is I must tranform a point cloud from wgs84 (geographic) to UTM (projected) ED50 for zone 31North. Easy, isn't it? Well, first surprise is I have several free webs who offer me this service, and the answer is slightly different on every one of them. Worse of all, I transformed it using ArcCatalog 9.2 ToolBox (the one you find on 'Data management tools->Projections and transformations->Feature->Project' and used it for the sake of Esri's good name. But finally I found a little excel tool on this link http://www.aristademidi.es/app/download/5793157100/Conversi%C3%B3n+GPS_UTM.xls (i think it can be credited to geographer Gabriel Ortiz from www.gabrielortiz.com , but this detail could be inexact) and quite to my despair, it turned out that the locations this tool gives seem more exact (when I draw it over my cartographic base) than the ones from Arcgis. The difference between this and the excel tool is systematically a bias +10m E.
On Arccatalog I used the parameters from WGS 1984.prj to European Datum 1950 UTM Zone 31N.prj, using something called transformation ED_1950_To_WGS_1984_31. On the excel tool I used the upper side of the second sheet.
So, where is absolute truth on this matter and where could I find the 'real' numeric transformations in case I must code a tool that I can really trust?