Thursday, September 29, 2011

Lars Rasmussen on the startup of Google Maps

Here's some interesting background to where Google Maps came from.  Must feel pretty good to have created a revolution (slippy maps) - and maybe to have just missed creating another one (Google Wave as an alternative to email).

Tuesday, September 27, 2011

Data model diagrams for GTFS

Recently I've been doing some work with the General Transit Feed Specification (GTFS).  This is a simple data model initiated by Google, in order to allow transit organizations to publish their schedules for routing and visualization purposes.

As usual, Google has nice, succinct documentation for the GTFS format (which is a set of CSV files embodying an implicit data model).  But I was surprised to find that they don't provide any kind of data model diagram showing the relationships between the various entities (files) in the model.  I have a feeling that they think this would be "too technical" for users - or maybe they just don't believe in using a long-established body of theory?

So, to help my understanding of GTFS I put together a simple diagram of the data model.  I used Google Diagrams, which is why this looks a little primitive.   (Hmmm... maybe another reason they don't do diagrams is that they are religious about dog-fooding?)

After doing this I found the paper A Transmodel based XML schema for the Google Transit Feed Specification, which has a much more detailed UML model of GTFS:

The end goal of doing all this was to be able to transform a GTFS data feed into shapefile format.  For this I used a simple JEQL script.  But that's another blog post.

Monday, September 19, 2011

Paul Ramsey's Keynote at FOSS4G 2011

As usual, Paul delivered a highly amusing and thought-provoking keynote at FOSS4G 2011.  For the benefit of the hard-core twitterless, the slides are here.

Thursday, September 1, 2011

Everything you wanted to know about UK Coordinate Systems

Those helpful folks at the UK Ordnance Survey have provided a fine reference Guide to Coordinate Systems in Great Britain.  It's well worth reading if you're interested in the intricacies of coordinate systems, datums, surveying and GPS.  It's a bit technical, but if you can handle coding in Javascript you can probably get something out of it...

An interesting extract gives the general flavour:
  • The WGS84 Cartesian axes and ellipsoid are geocentric; that is, their origin is the centre of mass of the whole Earth including oceans and atmosphere.
  • The scale of the axes is that of the local Earth frame, in the sense of the relativistic theory of gravitation.
  • Their orientation (that is, the directions of the axes, and hence the orientation of the ellipsoid equator and prime meridian of zero longitude) coincided with the equator and prime meridian of the Bureau Internationale de l’Heure at the moment in time 1984.0 (that is, midnight on New Year’s Eve 1983).
  • Since 1984.0, the orientation of the axes and ellipsoid has changed such that the average motion of the crustal plates relative to the ellipsoid is zero. This ensures that the Z-axis of the WGS84 datum coincides with the International Reference Pole, and that the prime meridian of the ellipsoid (that is, the plane containing the Z and X Cartesian axes) coincides with the International Reference Meridian.
  • The shape and size of the WGS84 biaxial ellipsoid is defined by the semi-major axis length a  6378137.0 metres, and the reciprocal of flattening 1/f  298.257223563. This ellipsoid is the same shape and size as the GRS80 ellipsoid.
  • Conventional values are also adopted for the standard angular velocity of the Earth, and for the Earth gravitational constant. The first is needed for time measurement, and the second to define the scale of the system in a relativistic sense.
Now then, if you find yourself getting lost on the way from Foyle's to the nearest Lyon's tea shop, you can't say you haven't been warned!