> First, the upcoming 1.61 release will have changes in overlay. Please check there if the problem still appears.

Ok, I will try it, thank you! (Forgot to mention, but I think you had understand that I'm using 1.60.0 release)

> Second, the JPG's are illustrative, but hard to reproduce the problem. Could you file a ticket with the WKT representations of two polygons where the union fails?

I create ticket https://svn.boost.org/trac/boost/ticket/12118

> WKT can easily be generated with the library using std::cout << boost::geometry::wkt(input1), etc

I used boost::geometry::dsv before, WKT return floats with more decimals, but not enough, points that enclose those "almost zero areas" are the same, so that area is totally empty, not "almost" (it is just an edge).
Is there any way to print floats in WKT representation with more precision?

I wrote in the ticket, but want to repeat here:
I forgot to mention that despite the fact that I use same initial data (vehicle path) every program restart, boost geometry union_ makes described problems not in same places.
So WKT representation in the ticket different from pictures in previous post.

https://pp.vk.me/c631319/v631319687/1f012/2e1MycdOkB8.jpg now in last program run is what it should to be - only one polygon: https://pp.vk.me/c633617/v633617687/1cacb/mI45Se8H4OI.jpg

Appended mpolygon is https://pp.vk.me/c633617/v633617687/1cab7/r_7Ep30w_jc.jpg
And union result now is https://pp.vk.me/c633617/v633617687/1cac1/bc2qTKY-8Bs.jpg

Thanks, Oleg