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I'm trying to transform a map from UTM ED 50 zone 33N to WGS84 and vice versa in ArcMap 10.2.2.

I'm doing this by using the Project tool in the Arc toolbox and choosing the ED_1950_TO_WGS_1984_12 transformation (since the area in question is Malta). However when I compare the completed transformation with another map of the same data in the same projection, there is an offset of 22m.

Can anyone please indicate why this is happening, and how I can improve the accuracy of my transformation?

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  • +1 because you actually included all the information in your question (which transformation you used and the area you are looking at). And this isn't quite so basic a question. We have a couple of projection/transformation experts here I'll leave a full answer to (because I barely understand), but referencing this pdf that particular transformation has an accuracy of 44 which is comparatively low to some of the others. While you picked based off the name and area suitability, another might be better.
    – Chris W
    Apr 27, 2015 at 19:51
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    Prof. Mugnier lists another transformation here: ΔX = –83.7 m, ΔY = –81.9 m, and ΔZ = –131.6 m.
    – mkennedy
    Apr 27, 2015 at 20:46
  • @mkennedy I was hoping you or AndreJ might chime in with a more accurate transformation to use, but it sounds like perhaps there isn't one included in software and a custom one, such as from the article you cite, would have to be created with the Create Custom Geographic Transformation tool and then tested to see if it's any better than 12 is? BTW, just to confirm my own understanding, that 44 in the accuracy chart means it's off up to 44m right? I might have missed it but didn't see explanation in the pdf.
    – Chris W
    Apr 27, 2015 at 22:10
  • We're cribbing the accuracy from EPSG who calculated it from NGA's separate offsets in XYZ. I haven't asked how Roger (EPSG) is calculating 1 number from NGA's 3! But, yeah, most tfms from NGA have poor accuracies.
    – mkennedy
    Apr 27, 2015 at 22:40
  • So it seems that offsets are probably a transformation inaccuracy right? Also many thanks for your comments and suggestions
    – Geoniette
    Apr 28, 2015 at 9:45

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Since you specified the transformation you used, I looked it up in some reference material Esri provides on selecting a transformation to use - specifically, this pdf which lists available transformations in ArcGIS. It includes an accuracy column. Although there isn't any specific explanation of the numeric value, and mkennedy has noted it is derived from a third party source via unknown calculation, I make the assumption that lower is better (and theorize it means a point after transformation will be within x meters of original location, x being the accuracy).

You likely chose the transformation you did based on the name and that it was designed for that area, but the chart indicates a relatively inaccurate rating of 44 compared to some of the others that are near or below 1. So while that is the best transformation offered by default, matching that name, another one might give better results - including a custom transformation.

mkennedy found an article by a Prof. Mugnier giving parameters for another transformation for that area: ΔX = –83.7 m, ΔY = –81.9 m, and ΔZ = –131.6 m. Those parameters can be put into the Create Custom Geographic Transformation tool in ArcGIS to generate a custom transformation file that can be applied instead of picking one of the defaults/included. These parameters may (and apparently they or another set have) provide a more accurate result.

The actual details of why that particular transformation has the accuracy it does gets into math and geodesy that's over my head.

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