The bend filter modifies each route replacing each point inside a curve with two points: one at a given distance in the direction of the previous point, and another at the same distance in the direction of the next point in the route. It only replaces points where there is a change in heading big enough.
When creating a route, points are usually created inside curves or intersections. That means that, while navigating that route using a GPS unit, the course pointer would aim to the inside of that curve or intersection, and only when you have passed that point will the GPS aim to the next waypoint in the route. This behaviour is useful in marine navigation but when biking, for instance, it may be a bit late to decide where to turn to in an intersection.
This filter tries to solve that creating a waypoint before and after where there is a change in direction.That way, the course pointer will point to the direction you should turn to ahead in time.
For this filter to work correctly, the route should be simple enough that there is only one waypoint inside each curve or intersection. Because of that, it is usually a good idea to use the simplify filter before this one.
This command line reads route.gpx and replaces each point with other two points: one 25 meters before and another 25 meters after the original point. It replaces a point only if there is a change of direction larger than 5 degrees.
gpsbabel -i gpx -f route.gpx -x
bend,distance=25,interpolate,minangle=5 -o gpx -F newroute.gpx
Distance to the bend in meters where the new points will be added.
Distance in meters to the original point where the new points will be added.
The new points will be created at this distance. The first one in the direction of the previous point, ant the second one in the direction of the next point in the route.
Minimum bend angle in degrees.
Minimum curve angle in degrees.
The substitution will only be made if the change in the heading is greater than this value. This avoids replacing a point if the GPS unit is already pointing in the correct direction, or if the route reaches a certain point and goes back the same road.