Hi Ticker, the patched version still returns ON for a Coord which is not ON and thus doesn't work with my example. The result doesn't depend on the position of the last point of the way as long as it is builds a nearly or exactly straight line with two nodes of the shape. Your algorithm returns true for each such point, even when it is 100m away from any shape vertex. See my new example where A,B and C build a straight line. Another node is very close to the edge but still returns IN (OK) Gerd ________________________________________ Von: mkgmap-dev <mkgmap-dev-bounces@lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap@jagit.co.uk> Gesendet: Donnerstag, 21. Mai 2020 17:07 An: Development list for mkgmap Betreff: Re: [mkgmap-dev] Explanation of the is_in function Hi Gerd Here is patch that prevents possible underflow when node is very close to an almost horizontal or vertical line and incorrect results when exactly on this line. Ticker