Tour MapOctober 24th, 2008
Well, now I know all about geographic datums and projections, and what’s the difference between WGS84, EPSG:4326, EPSG:900913. The answer is that WGS 84 is a datum, EPSG:4326 is a projection which refers to WGS 84, no hang on that’s not right, it’s a datum, the same one WGS 84. Actually I’m not so sure about that. Anyway, EPSG:900913 is the new name for a spherical mercator projection. Or is it a datum, I’m not sure? The important thing is that it approximates the Earth as a sphere instead of an ellipsoid, and this makes some geographers really angry.
EPSG:900913 is better known as the google projection and has the property that the world (most of it) becomes a square. The square can be divided into 4 smaller squares, and so on, and this is how the tiling works in google maps.
Unfortunately the projection suffers distortion which makes Alaska look bigger than Western Australia1, and for technical reasons Father Christmas’ house must be left off the map — this is really what makes the geographers angry.
(Each colour is a different day, the dots are where I took photos. The gaps are because of GPS problems. Train trips are from Venice to Vienna, and from Zakopane to Wrocław)
Behind the coloured lines are my dodgy collection of scripts which take NMEA data from the GPS, correlate them with photos and my written notes, and arrange them ready for MapServer. Languages in use are Python, Perl, awk, sed, m4, and a big nasty Makefile which ties them all together.
Anyway, that’s enough fiddling and reminiscing about bike tour for now. I hope you enjoy the map, bye!