Chains as a measurement.

[snowball]

Either way on a piece of land 0.5 furlongs square compared to 1ha you're talking about a 2.5cm border. So for most real-world applications it's 'close enough'.

Er, not quite. For an error of 0.5% in the sides of a 100m square, or an error of 1.0025% in its area, you need an error of 0.5m in each side, so a 25cm border.

 

[edwin_m]

Back on the topic of chains. I recall that a few years ago (probably as a result of the Grayrigg derailment when it became clear that Network Rail hadn't been very good at managing its track assets) there was uproar over the detail that Network Rail did not have a proper inventory of its assets. Apparently nobody in Network Rail could adequately provide an account of the total assets it was responsible for.

It was pointed out that this was partially due to the fact that over the years NR had used a whole range of different units to manage its tracks (probably a mixture of miles, kilometres, chains, metres and yards) which made it hard to keep track of things. The solution was to standardise on metric, the land the tracks are on is surveyed in metric and the trains that ride on them are designed in metric so it makes sense to use the same units.

I don't expect people with tape measures to walk the entire length of all tracks and do an entire stock take in metric. I do expect that any major maintenance (electrification? tamping? sleeper replacement? remodelling?) will involve measuring everything up in metric once and for all and stop depending on many decades old records. Especially with the advent of modern Geographic Information Systems (GIS) that should be fairly straightforward and more precise than ever. Imagine having a computer system that can tell you that the distance between an outer rail and the column of a particular bridge is precisely 2241 millimetres.

As I posted somewhere previously for signalling plans, this is exactly what happens for new ones but older ones probably won't be updated until whatever they show is either replaced or significantly modified. However this won't happen for all the assets on the same route at the same time, which is one reason why the signlaling plans I mentioned include the position of existing mileposts. This allows position to be aligned with existing features, and is valid even if the mileposts aren't a mile apart.

When establishing the position of items on the railway, discrepancy in units isn't a major issue as long as it's clear what units are being used. It's likely that much more confusion would arise if someone started measuring from a different zero point.

 

[edwin_m]

Whenever (if it's actually used on the railways) LIDAR is used, do all distances that it measures get recorded in metric?

LIDAR and other survey information really needs tying down to a specific location on the planet, which is a whole other area to think about. When I was testing some of the first GPS units on a train in the late 80s the position could be tens of metres out from the Ordnance Survey, because the two were based on slightly different models of the shape of the Earth (I suspect more recent models correct for this). Once you have all data on a common frame of reference than it's pretty easy to establish the distance between any two places in any units you care to mention.

 

[najaB]

Er, not quite. For an error of 0.5% in the sides of a 100m square, or an error of 1.0025% in its area, you need an error of 0.5m in each side, so a 25cm border.

Indeed - I divided by 10 one time too many. It's still not very much for most real-world applications.

 

[duffield]

Indeed - I divided by 10 one time too many. It's still not very much for most real-world applications.

I think there have been court cases about strips of land of around 25cm wide. It could be the difference between having a useable and non-usable driveway!

 

[AndrewE]

I think there have been court cases about strips of land of around 25cm wide. It could be the difference between having a useable and non-usable driveway!

It's called "a robber's inch." A friend of mine accidentally ended up owning one across the end of a cul-de-sac next to his house, and made a bomb when the field beyond was wanted for more housing development.

 

[snowball]

It's called "a robber's inch."

The phrase I've seen is "ransom strip".

 

[GRALISTAIR]

An Acre is a chain x a furlong.

In all my 60 years of life and being good at maths and knowing metric and imperial I never knew that - just checked -you are absolutely right - I learned something today. Thank you sincerely.

22 yards and 10 chains I n a furlong so 220 yards so 22 x 220 = 4840 sq yards = 1 acre

 

[snowball]

Indeed that was the typical dimensions of a plot of land that could be ploughed by a team of oxen in one day. A long thin plot was preferable to a square one because it takes time and effort to turn round on reaching the end of each furrow. (One furlong = one furrow long.)

 

[najaB]

I think there have been court cases about strips of land of around 25cm wide. It could be the difference between having a useable and non-usable driveway!

Surveying is not an everyday application though - which was the word I should have used rather than real world.

 

[philthetube]

So you're happy to work to 5% errors?

Just a question. When these enthusiasts built Tornado, did they get specially made imperial nuts and bolts? I seem to remember that the boiler had to be made in Germany, so presumably that was metric...

In all my 60 years of life and being good at maths and knowing metric and imperial I never knew that - just checked -you are absolutely right - I learned something today. Thank you sincerely.

22 yards and 10 chains I n a furlong so 220 yards so 22 x 220 = 4840 sq yards = 1 acre

It is a useful/interesting thing to know, if only because when you see a property for sale with x acre garden you can picture it.

 

[Joseph_Locke]

Surveying is not an everyday application though

A motto by which many of our survey subcontractors seem to live ... "Accurate? That'll cost you extra"

 

[Flying Phil]

A motto by which many of our survey subcontractors seem to live ... "Accurate? That'll cost you extra"

......and some fascinating new knowledge ....what a great thread!

 

[InOban]

Even, or perhaps because, it wandered a long way from trains....