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Written on 17/11/16 by Paul Oldham

Why Do Some Maps Appear Crooked?

A user of our GPX mapping software posted on our forum today. She said:

Hi - I've just found WalkLakes and it looks a great site. However, on my computer screen the maps are noticeably crooked. The horizontal grid lines slope down from top left of screen by about 10 degrees or so. More importantly, it prints crooked too. Is this a known problem or something peculiar to me? Thanks

The answer is that it's neither really, it's just how it is.

The issue here is that in creating the National Grid the Ordnance Survey treated Great Britain as being on a flat surface rather than a globe - as is illustrated in the left hand map1 below. The tapering lines are the lines of longitude and curved lines are lines of latitude on this projection of Great Britain.

When we are using our own map tile server (which we use for all the smaller scale mapping - the "road atlas" style zoom levels) or the Ordnance Survey's tile server (which we still use on our first GPX mapping site maps.the-hug.net) the tiles we're given are orientated to the OSGB grid, so the grid lines appear horizontal and vertical.

However things get more interesting when you're logged in. Then the tiles for 1:50K and 1:25K mapping are served from Bing's tile server. Bing doesn't care about the National Grid, it's a worldwide mapping system so it gives you tiles with lines of latitude horizontal and lines of longitude bent outwards so that they are vertical. So the OSGB map is distorted to fit that as is shown in the right hand map above.

So if, for example, you are at Cirencester, SP045016 which is close to the bottom left hand corner of the SP square then (as you might expect from that map on the right above) the grid lines are pretty close to horizontal and vertical:

But if you are at Glencoe, NN102588 then the grid is leaning to the left:

Similarly if you look at Lerwick, HU476413 then it's a bit cocked the other way:

The problem here, and one which humanity has had ever since we've had maps, is that we're making approximations to cope with the fact that what we're mapping isn't really on a flat surface but a curved one but we're trying to represent it on a something flat, be it a sheet of parchment or a computer screen. There are many ways of getting round this but every one leads to distortions of some sort.

Anyway, if your eyes are glazing over by now the tl;dr summary is:

yup, it does that for everybody, don't worry about it

  1. These maps are from this web page although I think the original images on which it's based are probably Ordnance Survey's.

Tagged: GPS, maps


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