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I have a polygon file in qgis3.0 representing global tectonic plates. I have reprojected the original file in WGS84 (EPSG:4326) to PDC Mercator (EPSG:3832) using only the features that are needed.

Because each feature in the polygon crosses the antimeridian, some (selected) have lines cutting through them:

enter image description here

In the original file the two largest of these features have just one attribute entry, while the other has two but with identical values:enter image description here attribute table for the reprojected polygon

There are probably a number of ways to remove these lines. So far I have tried Simplify and Polygons to Lines to manipulate the nodes but this just causes the outline to become deformed.

Screenshot of invalid geometries: enter image description here

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    Try Vector -> Geoprocessing Tools -> Dissolve, Uncheck Dissolve All, and select the Code field for dissolving. – AndreJ Mar 31 '18 at 7:21
  • This worked great for the two features that share attributes, however it hasn't dissolved the line between the larger two features. – Ewan Mar 31 '18 at 13:48
  • Trying your issue with the data from github.com/fraxen/tectonicplates I get invalid geometries after reprojection of the pacific plate only to PDC Mercator. – AndreJ Mar 31 '18 at 16:11
  • Yes it is the same data. I have tried this myself and get the same results for both the pacific plate and the australian plate, I have added a screenshot of this. I'm not sure if there is a way to divide these features and then dissolve them. I also have the original data in .dig format so it might be possible to create a new dataset which is not based on the wgs 84 but I don't know how this is done – Ewan Mar 31 '18 at 18:46
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    It works for me if I run Multipart to singlepart on the unprojected dataset, then reproject, then dissolve. – AndreJ Apr 1 '18 at 13:44
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As @AndreJ says: Multiparts to singleparts > reproject > dissolve

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