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I've got some data that is in WGS84 UTM 29N and some that is in WGS84 Complex UTM 29N.

The WKT for the complex projection looks like this:

PROJCS["WGS_1984_Complex_UTM_Zone_29N",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator_Complex"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-9.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0],AUTHORITY["ESRI",102579]]

I reprojected both sets of data using FME to EPSG:3857, and the Complex UTM 29 data ended up 1500 KM southeast of where it should have been. My guess is that FME didn't know how to handle PROJECTION["Transverse_Mercator_Complex"] because all of the other parameters are identical to the regular UTM 29 data.

Now I'm curious as to the difference, so I started searching around.

  • I searched the EPSG database and couldn't find a CRS match searching on "complex."
  • No good results on the first few pages of a Google search
  • Nothing obvious in the couple of map projection books I own

What is the Complex Transverse Mercator projection?

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    Search with also "ESRI" as a keyword desktop.arcgis.com/en/arcmap/latest/map/projections/… A new implementation called Transverse_Mercator_complex has been added to the Projection Engine which is available in ArcGIS. It accurately projects to and from Transverse Mercator up to 80° from the central meridian. Because of the more complex mathematics involved, performance is affected. I would say that edit the WKT, remove _Complex, and you should be fine with FME.
    – user30184
    Feb 1, 2017 at 18:30
  • Thanks! I scanned that page twice and completely missed that blurb. If you can post this as an answer, I'll accept it.
    – Mintx
    Feb 1, 2017 at 18:40
  • Esri employee here: when we implemented the 'complex' version (for Scandinavia mainly), we didn't want to replace the existing TM method because the complex one is a lot slower.
    – mkennedy
    Feb 1, 2017 at 21:15
  • @mkennedy Scandinavia? Why do they want to spread some TM or GK projection up to 80° from the central meridian if zones are usually 1-6 degrees wide? I would understand better if it deals with high latitudes up to 80° from the equator. Or do they want to handle the whole Scandinavia in the same zone?
    – user30184
    Feb 1, 2017 at 21:33
  • They didn't but they were using wider than 6° of longitude which didn't unproject precisely at high latitudes at the time we added the complex version. Plus there was some wide zone usage in Greenland. The 80° is from the developer pushing the edges of the algorithm. The standard TM algorithm now also unprojects precisely up to 45° from the c.m.
    – mkennedy
    Feb 1, 2017 at 21:38

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Transverse_Mercator_complex is explained in http://desktop.arcgis.com/en/arcmap/latest/map/projections/transverse-mercator.htm

A new implementation called Transverse_Mercator_complex has been added to the Projection Engine which is available in ArcGIS. It accurately projects to and from Transverse Mercator up to 80° from the central meridian. Because of the more complex mathematics involved, performance is affected.

It feels somewhat wrong to call that as a different projection in WKT because the projection is the same Transverse Mercator and complex stands for the more complex algorithm. However, ESRI WKT is made for ESRI products and I suppose that they know what to do even FME didn't.

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I think it depends on how you reprojected this in FME. If you used a plain Reprojector transformer - or the reader/writer coordinate systems parameters - then yes, it could be a problem.

What FME does is read the WKT and try and match it to a coordinate system it recognizes. In this case it doesn't recognize those "complex" coordinate systems, hence the problematic output.

But if you use the EsriReprojector transformer, then we'll pass the coordinate system information straight to the Esri reprojection engine without trying to match it to anything else.

So in that case you should get the reprojection you need.

The caveat here is that you would need ArcGIS installed, because that's how we gain access to the Esri reprojection engine.

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