Thursday, 10 September 2009

From A to B - But Where is B?

A long-standing friend rang me up today with a geo connudrum. OK, it may be obvious to land lubbers but for us hydrographers puzzles like this confuse us...

When road signs say x miles (or km for my more enlightened readers) to a town/city, where is the the point to which they are measuring?

Ie, I know where A is but where's B? It's pretty straight-forward for a standard cute British village with the village green, church and pub (you can find the middle +/-100m easily) but where is the centre of London, Sydney, Paris? Or are road signs a bit of a hit and miss affair?

Answers on a blog please...


Kevin Nehring, PLS said...

Interestingly, I was asked this very question just a few weeks ago. It was the first of two related questions actually.

Here in the States, within the past century and a half, the "center" of town was what was measured to. That "center" was not the physical center, but the community center, usually a church or hall that was used for town meetings. When the town grew large enough to get its own Post Office, that Post Office usually became the town "center", and the point measured to. It was a physical, and Federal, location that identified the community.

As the communities and towns grew into larger cities with multiple Post Offices, the city limits became the measured point of reference. One of the problems with using the city limits is that the signs referencing to them are not changed as often as the city boundaries. Most of the signs are maintained by the State or County highway departments, which move at a much slower pace than the growth of a city. A sign may state a particular distance to a city - and probably has for the past 50 years - but that city is certainly no longer in the same location. At least its boundaries aren't. It will probably be a while before the sign gets updated too, because there's a 50+ year-old map filed somewhere in the highway department that says that THAT sign on THAT stretch of road is to state THAT particular distance to THAT city.

I know of one local sign that states a distance of 9 miles to the city up the road, when in fact, the sign is now within the City limits. I always laugh at when I see it.

The second question I was asked was this: How does a mapping program or website, (like Google Earth, Google Maps, MapQuest, etc.) determine where a particular town is. My answer: "I have no idea".

Ruth said...

Thanks Kevin. I loved your last sentence ;-) Sorry for being late posting this up - I've been on leave and your comment got lost in the e-mails somewhere.

Ken said...

Following on from Kevin's comments, I think that we use a similar system in UK. Years ago, I seem to recall, distances derived by the AA were, for London, always referred to Trafalgar Square. Presumably, therefore, they would, for Bath for instance, be applied to the Abbey area, and so on.