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Bezier Curves – A More Flexible Alternative to Great Circles

This tutorial introduces a flexible method for generating flight paths in PostGIS using a custom bezier curve function. Unlike Great Circles, bezier curves provide the user the ability to set the amount of ‘flex’ or ‘bend’ exhibited by the curve, the number of vertices (useful for timeseries animations), and a break value at a predefined meridian to ensure the visual integrity of the curve is maintained when plotting in your favourite projection. I also have a 3D version of the bezier curve function which I’ll share at a later date.

The full code for this tutorial, including the QGIS technique for centering a world map in the Pacific Ocean, is available on github here.

flightpaths

I won’t repeat everything which is already documented in the github tutorial code and custom bezier curve function, except make mention of the five arguments used by the bezier curve function itself.

DE_BezierCurve2D(origin geometry, destination geometry, theta numeric, num_points integer, breakval numeric)

  1. origin geometry
  2. destination geometry
  3. theta, which is the maximum bend in the East-West plane
  4. the number of Linestring points or vertices
  5. the break value which splits the Linestring into a Multilinestring at a specified meridian

Example: DE_BezierCurve2D(o_geom, d_geom, 25, 1000, 30)

Origin and destination geometries should be self-explanatory.

Theta defines the maximum degree of ‘flex’ or ‘bend’ in the East-West plane, to which a SIN function is applied to produce the effect of straighter lines in the North-South plane. If you prefer that all curves exhibit the same degree of bend irrespective of the azimuth between their origins and destinations, then modify the bezier function by replacing SIN(ST_Azimuth(ST_Point(o_x, o_y), ST_Point(d_x, d_y)) with the value ‘1’. Or if you want to stipulate a minimum bend in the North-South plane, then enclose the associated formula within a PostgreSQL ‘greatest()’ function.

The number of Linestring points or vertices is a really handy feature for animating flight paths where you need to synchronise movements along flight paths using a clock. To do this, set the number of points equal to the flight duration between each origin destination pair. So a 60 minute flight gets 60 points or vertices. A 120 minute flight gets 120 points or vertices. Then dump the flight path vertices into a points table which you can then index to a flight timetable.

The last argument is the break value, which is the meridian at which the flight path will be broken into two Linestrings (thus becoming a Multilinestring). The break value should equal the PROJ4 ‘+lon_0’ value associated with the CRS which you intend to view the data in, plus or minus 180 degrees. An example is given in the github tutorial file.

At some point I’ll release my 3D version of this function which is very cool for making 3D flight path animations.

Enjoy!