diamond geezer

 Thursday, June 16, 2005

But which way is West?
"One thing I note from your Geoblogging line: The line appears to be slightly rising (ie: not totally west?)... this I don't understand." [NiC]
"I was wondering if this is because true north is not (quite) the same as grid north." [dg]
Take another look at my Geoblogging map. Yes, the line of photos tracking my "Go West" walk really does appear to rise slightly as it crosses the map. You'd expect a westward walk to head precisely horizontally, but it doesn't. Why should this be? I've been digging around the internet and I think I've found the answer. Deep breath now.

Reason 1: There are three different types of north.
i) True north = the top of the axis about which the Earth rotates.
ii) Magnetic north = the point on the Earth's surface towards which all compasses point. Near enough, anyway. The Earth's northern magnetic pole lies in the Arctic Ocean somewhere around 84°N 115°W. More info here. It's been heading steadily northwards over the last century (maps here), and scientists reckon it's just (literally just) left Canadian territorial waters. Being fast-moving and off-centre, magnetic north isn't the most reliable means of navigation, not unless you know how far off centre it is. I've used a handy online calculator to discover that, here in Bow, east London, magnetic north lies approximately 2½° west of true north, decreasing very slowly year on year. But I wasn't using a compass on Monday, so that's not why I was walking in the wrong direction.
iii) Grid north = the direction of a vertical grid line on the UK National Grid. And here's the problem. The Earth is curved but maps have to be flat, so there's some distortion projecting one onto the other. See the UK maps here and spot the difference. The only places in the UK where grid north exactly matches true north lie along the 2°W line of longitude - that's a line down the middle of the country through Aberdeen, Birmingham and Jersey. For all places to the west of this line grid north is slightly to the west of true north, and for all places to the east of this line (including London) grid north is slightly to the east of true north. Here in Bow, for example, my local OS map tells me that the difference is about 1½°.

Reason 2: There are, therefore, three different types of west.
i) True west = a line parallel to the equator.
ii) Magnetic west = a line at right angles to magnetic north. Which is actually quite a freaky concept - take a look at the grid lines on this map.
iii) Grid west = the direction of a horizontal grid line on the UK National grid. And I planned my walk on Monday using an Ordnance Survey map, so my walk travelled grid west rather than true west. As with grid north and true north, again there's a 1½° difference between the two here in London. It doesn't sound much but, after my 12 mile walk, I actually ended up 600m further north than I should have done. The Hoover Building (here) may be grid west of my house, but had I been travelling true west I would have ended up on the other side of Ealing Golf Course at a non-descript footbridge over the River Brent (here) instead. Damn, missed.

Reason 3: There's more than one type of mapping grid.
i) The National Grid is a UK invention, designed to keep mapping inaccuracies within the UK to a minimum. Complete details here. In this age of global navigation, however, a national grid isn't much use...
ii) ...and so GPS devices use another grid called WGS84. This takes account of the fact that the earth isn't a perfect sphere - it bulges slightly at the equator - so the best global grid isn't a sphere, it's an ellipsoid. Google Maps uses location data based on this global egg-shape (where grid west is much closer to true west), whereas the Ordnance Survey uses a flat grid instead (which, here in London, is approximately 1½° out of true).

And that, NiC, is why my westward walk (planned on UK maps) rises very slightly on the Geobloggers map (which uses global data). It's because I wasn't walking true west. Bugger.

Simple moral of the story: Unless you live in Aberdeen, Birmingham, Jersey or points inbetween, map north isn't true north and map west isn't true west.
Even simpler moral of the story: Don't trust maps.
Conclusion: No, I'm not doing the whole walk again.


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