The video below shows an interesting situation with linestrings. The question was raised by a company using Leica Stringless pavers. They reported that some linestrings (typically closed linestrings) when you convert them to alignments, have an extra segment in the vertical alignment that starts at below 0 station and leads into the horizontal alignment that starts at 0 station. The Leica Pavers cannot cope with alignments that have a different HAL and VAL start point. While geometrically this is legal, and while LandXML supports it, the Leica pavers apparently do not like it. You can also see this when you open the linestring in the Profile Viewer.
But when we reviewed the issue and discussed the fixes that the customer has to apply, they mentioned two solutions
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Break the closed line half way down a straight segment, and then rejoin the segments making the break point the new start point of the linestring. While this is actually correct and it does work, it is not specifically the breaking and moving of the start point that cures the issue. When you break the linestring, it adds VPIs to the two line segments that it creates at the same location. When you rejoin the lines, the start and end point of the line now have elevation, and that is in fact what cures the issue.
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Put a small break in the linestring between the first and last point of the line, so that it is no longer closed. Again this is doing the same as above. When you break a linestring, TBC auto interpolates the Z value at the break point and adds VPIs at that location so that the geometry is maintained. If the break point is in the middle of a vertical curve, TBC also creates a phantom VPI off the end of the alignment so that the vertical curve geometry is maintained.
But in reality the issue in this case turns out to be as follows
The Linestring in question is a lozenge shaped island. It starts at the beginning of one of the straight sections and the first segment is a straight. It then goes through an Arc - Straight - Arc Straight - Arc - Straight - Arc combination to close back onitself. The linestring has VPIs at all of the nodes with the exception of the last node where the last arc joins back to the start point. And this is actually the issue. When you have a closed line where the start and end point are the same, then the line will have the same elevation at the start and end point, however in this case only the start point has an elevation. If you add an elevation in the Horizontal Tab of the last arc at the arc end point, that matches the start point elevation, the problem is corrected. TBC no longer adds that extra segment in the profile Viewer and that is no longer passed to the VAL of a HAL/VAL created from the linestring and that no longer passes out in Landxml for the Leica pavers and the problem is fixed.
The TBC team have made a modification to the program to improve this work process for the future so you need not worry about this issue at all.
Video shows what I am talking about