Month: May 2019

Generating GPX and KML maps with Ruby on Rails

~ Paul Bogard ~ Leave a comment

While working on my various projects, I’ve dealt with various types of maps.

My Flight Historian plots flight data using the Great Circle Mapper tool. These maps are simple to generate from my flight data (I just have to pass it a plain text collection of airport codes) and easy to embed in my website. However, because they are static images, they can’t be easily panned, zoomed, or otherwise manipulated in the way that modern map websites and apps can.

A sample Great Circle Mapper map of my flights in 2018.
(Map generated by Paul Bogard using the Great Circle Mapper – copyright © Karl L. Swartz)

On the other hand, my driving maps require too much detail for a single image, so I create them in Google Earth, which lets me manipulate the view as much as I need to. The driving data is a bit more complicated than my flight log data; while my flight log represents the abstract shortest distance straight line between two airports (and thus only requires specifying the airport at each end), a single drive can involve tens of thousands of coordinates that can be joined together, connect-the-dots style, to show the actual driving route taken.

A sample driving data map using Google Earth

Fortunately, all those coordinates are automatically generated and saved by my car’s GPS navigation unit in a file format called GPX (GPS Exchange Format), which is an XML-based file format which contains (among other things) latitude/longitudes sampled, in the case of my particular GPS, once per second.

Sample from a GPX file from a recent trip

Google Earth doesn’t use GPX format (though it can import it); instead, it uses a format called KML (Keyhole Markup Language, from back when Google Earth was Keyhole EarthViewer). KML is also an XML-based format, so conceptually it’s similarly a collection of coordinates that can all be joined together, with its own slightly different style.

The same set of latitudes, longitudes, and altitudes in KML format. (Note that KML reverses the order of longitude and latitude.)

But while GPX and KML can be used to represent complicated route shapes, they don’t have to be. These formats are both just as capable of taking a pair of points on the globe and drawing the shortest line between them. With that in mind, I decided to try to have Flight Historian automatically generate KML and GPX versions of my flight map, which would let me show my flight routes in Google Earth and Google Maps.

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