I must tranform a point cloud from wgs84 (geographic) to UTM (projected) ED50 for zone 31North.
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'.
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.
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?
Best Answer
Whenever you have to transform or convert data between two geographic coordinate reference systems (GeoCRS, also called datums), you may find that there are zero, one, or multiple possible transformations available.
If there are multiple transformations, they usually differ by area of usage and/or accuracy. Some countries like Belgium have published several transformations that cover the same area but with improved accuracy as more data has become available.
For other Geographic CRS, there may be multiple ones for different areas, plus some with different accuracies. Some places to find out what's available include spatialreference.org and epsg.io. Please be aware that a good portion of the information at these two websites originates from the EPSG Geodetic Parameter Dataset, hosted by IOGP (International Association of Oil and Gas Producers).
In this case, the area of interest is in Barcelona - Catalonia. Checking for that area and transformations between ED50 and WGS84, finds ED50 to WGS 1984 (28), EPSG::1633 (in Esri terms, ED_1950_To_WGS_1984_28) which was originally published by CNIG and has a listed accuracy of 1.5 m.
Disclosure: I'm on the Geodesy subcommittee that maintains the dataset, and I work at Esri.