I recently shared a few beers with an old university lecturer. We noted the ever improving access to the excellent Ordnance Survey mapping data but tongue in cheek he pondered…
Here is a guest post from University of Edinburgh GIS lecturer, Bruce Gittings…
I see multimap have become the first to offer OS data on a slippy map
interface to a web service.
The writing is certainly on the wall for anyone contemplating a system which isn’t slippy – - that’s now the benchmark, anything else is dead technology. We’ve known space is continuous for a few years now and the average GIS toolkit gave up on ‘coverages’ and ’tiles’ a while back.
At the same time multimap also seem to have moved to using latitude and longitude, instead of eastings / northings on the national grid, probably as a function of the software they are using. The map is actually skewed to the window – most unusual. So, as more and more applications want WGS84 / GPS compatibility will the OS grid reference disappear? For decades this has been the de facto (indeed virtually the only) referencing system use for locating places on maps in the UK (excepting one-off grid systems used on town plans for example), despite a lat-long grid appearing subtly on the edge of OS sheets. Yet, the dash to lat-long driven my pan-national web services seems to be quickly moving the amateur place-referencer quickly towards lat-long too, if the dreaded wikipedia is anything to go by (wash my mouth out with soap for mentioning that word!). I wonder if they realise the implications?
There’s a lot more misunderstanding about lat-long than the good old simplicity-itself national grid. The oft-used precision (e.g.
55.12345678901234556 degrees north) – just because the computer can calculate to that number of decimal places – takes positioning to a ridiculous sub-atomic level. Even multimap – who should know better – have fallen into that trap. Also, most people don’t understand it is not possible to convert between lat-long and eastings/northings terribly accurately, and that, unlike eastings and northings, two numerically identical lat-long positions don’t necessarily refer to the same point. But let’s not pretend I’m a mathematical whizz who actually understands coordinate systems and transformations, but its all here if you want the details: A guide to coordinate systems in Great Britain.
But all of that only slightly undermines multimap’s achievement. I wonder if they had to get special permission from the OS to use their data in this way? It makes OS copyright a little easier to infringe, but also again (like google’s first) shows the remarkable usability of the slippy-interface and its underlying AJAX technology. This ‘blogger’ particularly likes multimap’s feature which identifies the ‘locality’ which continually updates as you move cross the map. They must have built a load of theissen polygons from the OS gazetteer to underpin this. The result works well in some places, but there’s no sign of the name on the map in others. Less good is the use of getmapping aerial imagery, which seems to have a lot of gaps, many more than it should have – people’s map wins there. And performance seems to be a struggle, you need big banks of powerful servers before you can offer a professional slippy / AJAX service – the demands of the clients are probably two orders of magnitude more than the previous technology.
Good try though – 6 out of 10.
Above Cartoon by Peter Steiner. The New Yorker, July 5, 1993 issue (Vol.69, no. 20) page 61