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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?

Thanks

  • I count at least 7 countries that fall into ED50 UTM 31N area of usage. That's possibly why the results may differ--different transformations being used. What is the area that you're converting? – mkennedy Nov 13 '14 at 17:44
  • btw, the #31 transformation is for Egypt. The number is just a variant number. It has nothing to do with a UTM zone! See the EPSG registry for the possibilities. Disclosure: I'm on the subcommittee that maintains it. – mkennedy Nov 13 '14 at 17:52
  • Hi and thanks, mkennedy. First of all, I'm working on the area of Barcelona - Catalonia. An example of the coordinates I'm using is (wgs84) N41 23.492 E2 11.230. This particular coordinate translates to (using Arcgis) (UTM ED50) X:432130.226, Y:4582745.084 and (using the above excel tool) X:432140, Y:4582746. – LoRaier Nov 14 '14 at 13:07
  • Based on that, try #28 which is published by CNIG with about 1.5 m accuracy, EPSG:1632 for the original (to ETRS89) and EPSG:1633 (to WGS84). – mkennedy Nov 14 '14 at 13:23
  • I want to test a few more points, but #28 seems to work like a charm, inside those 1.5m accuracy. Really, thanks a lot. I've never before had to transform coordinates from wgs84 and I'm still surprised about the lack of information on Esri's docs about those little files that seem to contain the geodetic parameters and turn out to be so important. Please, do answer my question and I will mark it as correct, or if you are too busy, I'll do it myself on a few days quoting your comments. – LoRaier Nov 19 '14 at 9:02
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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.

  • I've frequently found that IT staff is unaware even of the basics of reference systems, but this time it was I who was caught completely off-guard by this subtlity and got confused using a wrong transformation. Thanks again for that answer! – LoRaier Nov 28 '14 at 10:58
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I have found this WPS service to do that at the 'Institut Cartogràfic de Catalunya'.

Or, you can see all their services here (they have a REST service to convert coordinates).

Hope it helps.

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