21st century navigation
At the dawn of time Man navigated by the stars. Woman navigated by rolling down the window and asking a passer-by. But no more. Because earlier this week Amazon.co.uk delivered to Man the GPS device he’d ordered the week before.
This is a wonderful gizmo. Only this morning I used it to find my way from my front door to the station, a journey for which I previously had to rely on my memory. It’s easy to read the screen while walking along, and I’m sure that if I’d have concentrated on it just a little a bit more then it would have told me about the car which tried to run me down as I stepped into the road.
So I’ve not had a chance to go geocaching yet. Maybe this weekend. I have, however, uploaded to it about 60 local geocache locations as found at Geocaching.com.
I’ve also finally worked out what the point of map datums are. According to the Garmin site:
A datum is a mathematical model of the Earth which approximates the shape of the Earth, and enables calculations such as position and area to be carried out in a consistent and accurate manner. [...] Lines of latitude and longitude on a map or chart are referenced to a specific map datum. Every chart has a map datum reference. [...] If you are comparing GPS coordinates to a chart or map, the map datum in the GPS unit must be set to match the chart or map’s datum for accurate comparison.
Which is nice, but the device offers you the opportunity to change the datum, and I wondered what would happen if I changed it from the default (WGS 84) to, say, the GB Ordance Survey datum. The effect of changing it on the device wasn’t obvious, and certainly co-ordinate formats didn’t change from, say “N 51° 30.537 E 000° 05.586″ to “TQ 45351 80973″. But on closer examination a change in the map datum did change the actual numbers in the location. So “E 000° 05.586″ became, say, “E 000° 06.037″.
A map datum determines how the co-ordinates are worked out from the satellite information. A different datum actually gives different co-ordinates for the same spot — all to do with how you deal with the curvature of the Earth. It doesn’t matter unless you’re comparing what your device says to the co-ordinates from another source — an Ordnance Survey map, for example. Or you could just ask someone the way.
I’ll give an update on more GPS adventures as they happen.
