I wrote a delphi program generating a gpx file as input for a "poor man's guidance system" for aerial spray by means of ultralight plane.

By and large, it produces route (parallel swaths) using gpx file as output.

The route's engine is based on the "Vincenty" algorithm which works fine for any wgs84 computation but
I can't get the accuracy of grid generated by ExpertGPS of Topografix (requirement).

I assume a 2D computation on the ellipsoïd :

* 1) From the start rtept (route point), compute the next rtept given a bearing and an arbitrary distance (swath length).
* 2) Compute the next rtept respective respective to previous bearing (90° turn) and another arbitrary distance (swath distance).
* 3) Redo 1) with the last rtept as starting point but in the opposite direction, and so on.

What's wrong with it ?