should use a const reference for the parameter

Sign of divisor is discarded here, is that ok?

please add copyright headers with the usual license bla and your name to each file

let's try 2.8.12 instead

the code assumes that m_objects contains only placemarks in various places. It's better to reflect that in m_objects and change it from QList<GeoDataObject*> to QList<GeoDataPlacemark*>. If that violates the universal use of filters, introduce a convenience method for it.

I'd refactor it to have a private member QList<GeoDataPlacemark*> in LineStringProcessor that gets populated with document->placemarkList() in the constructor. I'd also store the document pointer in a private member variable, in my opinion protected member variables are a no-go.

Please update the usage instructions above to reflect the additional .shp support, e.g. using

parser.addPositionalArgument("input", "The input .osm or .shp file.");

This section seems to be the only difference to Marble's ShpRunner. I'd suggest to use that directly (like OsmRunner is included) and then postprocess the file (the resulting GeoDataDocument) and add the additional tag there. This avoids the copy duplication.

If you do not use any plugin internal API, you can also avoid compiling the runner at all. Just use Marble's parsing API:

MarbleModel model;
ParsingRunnerManager manager(model.pluginManager());
GeoDataDocument* document = manager.openFile(inputFileName);

IIRC BaseClipper shares a lot of code with ClipPainter. In such cases please add the original authors as well (just copy all Copyright... lines from the file and put them on top this one)

Manual polygon closing should not be needed anymore with http://commits.kde.org/marble/39fb2c303bc182d1d513c453c6233e5d1a341635