Spline is a QGIS plugin for digitizing splines as a chain of straight lines. It comes with a Processing algorithm for converting lines into splines.
A modified cubic Hermite interpolator is used to obtain a continuous piecewise third-degree polynomials between knots (known spline points). Each piece is converted to a chain of lines which is then simplified with Douglas-Peuker algorithm.
Both, digitizing map tool and the Processing algorithm, have the following parameters:
- tightness or tension - can be interpreted as the length of the curve tangent at digitized points, must be in interval [0,1]. See picture above.
- tolerance for Douglas-Peuker simplification algorithm - the smaller it is, the more segmented is the resulting linestring.
- maximum number of line segments - initial number of line segments interpolated between knots - these lines are then simplified.
The algorithm supports z and m values - they are interpolated linearly between knots, if properly defined in input layer.