Trimming 3D polylines to a shape

[unearthed]

I have an accurate contour data set but want to make it smaller for export to a GPS, so how do I consistently trim the polylines off outside the green polygon - the split tool seems to work on some lines but not others, it's a poor version of AutoCAD's fence tool that's for sure. I managed to trim 50 line in one go a couple of time but the tool seems to stop working /only work on singles.

 

I can do this in QGIS but it'd be nice to save the roundabout trip. I don't want to build a model in VW as I want get contours back in the same place as they appear now. 

 

System is VW2018.

 

 

[unearthed]

I found it works slightly better if switching to Ortho view but again fails after one run of the split tool.

 

So gave up and did it in BricsCAD really easily, with zero failure instances.

 

Something else I noticed using the split tool in VW was that the filesize kept growing (from 2.7mb to 5.5mb), whereas trimming in BricsCAD and reimporting that edited data took my filesize way back down again. Does VW not clean itself well after some operations? 

 

Lesson fir me is to do anything 'complicated' at least re topo data in QGIS or an ACAC-like platform.