Trajectory forecast vs track flown: a case study

During the recent Sagrantino Cup event I created daily trajectory forecasts with the windy.com traj plugin for the morning flights and asked pilots to share actual GPS tracks so I could compare actual versus forecast. Here is one example:

ECMWF forecast versus actual track – click to enlarge

The KML file for this screenshot is available here, and it is instructive to play with it – have a look at it yourself in Google Earth.

  • Most of the flight was between 800m and 1000m altitude, so I selected the 2000ft (amber), 2500ft (red) and 3000ft (blue) trajectories.
  • The ECMWF model had the best match, so only ECMWF forecasts are shown.
  • The flight started on the valley floor, within an inversion and a slight northerly flow.
  • The match is excellent – especially with the 2500ft trajectory, and the descent phases around 11km and 15km into the flight show the northeasterly flow forecast by the amber line.
  • As to be expected, the track is shorter than the forecast – slightly more than half the distance.
  • my gut feeling after comparing several forecasts to tracks: ECMWF wind speed forecasts are a tad on the high side.

Here is a second example:

  • trajectory colors: red: 2500ft, blue: 3000ft, purple: 5000ft
  • most of the flight was 1000-1200m altitude
  • the track’s match with the 3000ft trajectories (all models) again is very good!

Creating wind trajectories with windy.com

Update: head over here for the current version of the trajectory function – the text below documents plugin versions below 0.5.1.

windy.com is a phenomenal resource for weather forecasting. It’s less of a “will it rain at X by Y” type app but rather a data viewer for many different layers of information – you will have to draw your own conclusions. It also sports a variety of forecast models (among them the $$$ ECMWF) – for free. And it supports user-written plugins for specific purposes, which has unleashed a lot of creativity by weather-affine geeks.

For balloonists, the traj plugin is very useful – it can create tracks to answer the question “If I start at X and fly level in altitude Y at time Z, which track would I fly?”. And it can answer this according to several weather models – in Europe that’d be ECMWF, GFS and ICON-EU.

Here’s a short how-to for generating trajectories:

  • The traj plugin currently only works on desktop browsers – not (yet) on mobiles
  • The plugin must be explicitly loaded after pointing your browser to https://www.windy.com/ : click on the top left “Hamburger” (three horizontal bars) icon. In the dropdown, click “Install Windy plugin” and scroll down until you find the “Trajectory” plugin – then click on “Open”.
  • two windows will appear – one with explanations and one for parameters.
  • Select altitudes and duration in the parameter window.
  • Set a start time using the time selector at the bottom of the screen.
  • Select a model (ICON, ECMWF or GFS) in the right bottom model selector.
  • Choose a start location – either by clicking on the map window (the “picker”) or choosing a favorite by clicking the heart icon in the parameter window which will present a dropdown of your favorite locations
  • click “Start” – which changes the label on this button to “Stop”
  • the progress can be seen on-screen as the curves are drawn
  • the black bullets denote hourly intervals
  • Click on the lines, or the bullets for detail information (time, speed, model, model generation time etc)
  • when the plugin is done, you can download the trajectories as GPX files by clicking “Save”
Trajectory tutorial video (slightly out of date)

Advanced features include:

  • simulation of ascent and descent – if set to zero, just level flight will be simulated
  • backwards trajectories – those answer the question “If I want to go to location X, where do I need to start?”
  • duration of the trajectory: up to 240 hours – useful for gas balloon flights

For a discussion of the features and for reporting errors, see the Windy community website topic on the traj plugin.

For the curious: an example GPX file generated by the traj plugin

For geeks: here is the plugin code.

Creating tracks for Google Earth from GPX files

Google Earth is a great viewer for flight tracks. While GPX files can be imported into GE, they do not look appealing – and lack useful information.

Enter gpsvisualizer.com: a free online service to convert GPX files to the GE KML format (among many other features). I mostly use the Convert your GPS data for use in Google Earth service to visualize flights with these settings:

  • Google Earth doc name: start, destination & day of flight
  • Colorize by: Speed
  • Altitude mode: Absolute (for flights)
  • Draw a shadow: 30% opacity – yields an actual track over ground

Tip: click Save these settings and next time all settings will be restored.

Gpsvisualizer.com parameter settings

The result is a compressed KML file which looks like so when openend in Earth:

Update:

Many GPS sources create lots of trackpoints – so much that the resulting tracks look like a series of balls. Way too dense to be useful.

Also, the line segments generated by the settings above lack useful information (just text like ‘trkpt 660’). Adding waypoints in gpsvisualizer adds more detail.

Hour marks, as well as time stamps for takeoff and landing time are lacking above.

A good setting to deal with these issues is:

  • “Trackpoint distance threshold” : 20m
  • “Draw as waypoints”: Yes, named with timestamps
  • “Tickmark interval”: 1h
  • “Zero distance mark”: Yes (this field only appears when you enter a value into “Tickmark interval”)

This gives us waypoints every 20m (good enough) and klicking them in GE reveals useful information:

Clicking a waypoint. Note timestamp on takeoff location

The updated result file can be downloaded here.