Architects, builders, stonemasons? What's this symbol?

I’ve found this symbol in the stonework of several old sandstone buildings and have always presumed that it marks the presence of a drain or waterway. Am I correct? If not, what’s it mean? Thanks in advance.

its a baseline isnt it? i.e. a level from which other things are measured?
EDIT:
http://en.wikipedia.org/wiki/Benchmark_(surveying)

WTF

Predators exist?

datum mark/level i think. it would normally (in modern timber/steel/concrete framed construction) have the level noted next to it but i’d imagine they just keep a record in stone construction rather than scribe the numbers into the stone.

stoner is correct, every trade uses the same established datum in order that all their respective works tie in at the required levels.

Crows Feet.

It was carved by OS and is a surveyed level relative to AOD, a place somewhere in Devon IIRC.

There is a page on the OS website that indicates all of the locations and their level.

Usually found on old stone bridges or public houses.

ignore me.

I have been educated! Thanks very much, gentlemen.

A little late but an Ordnance Bench Mark (OBM)

http://benchmarks.ordnancesurvey.co.uk/pls/htmldb/f?p=111:3:9644676237886035422::NO:3::

and don’t move them as you need planning permission first 😉

I have been educated!

Me too – I didn’t realised that they weren’t used anymore, although I guess it’s not surprising with advent of GPS. Also I’ve never understood what defines sea level, surely it’s dependent on the tide – which can vary a fair few feet.

I’ve seen surveyors use them with theodelites, so I guess it’s still handy as an established reference point from existing buildings.

[Quote]ernie_lynch –
Member
I have been educated! Me too – I didn’t realised that they weren’t used anymore, although I guess it’s not surprising with advent of GPS. Also I’ve never understood what defines sea level, surely it’s dependent on the tide – which can vary a fair few feet.

[/quote]

Bless!

Altitudes are given in metres above ASL, which stands for AVERAGE sea level.

Something new for me too then…never heard “average” mentioned. Mind you, I hadn’t though about it…altitude never really being something I think about that much. 🙂

Well average doesn’t sound very precise, specially as sea levels are currently rising – every few years those bench marks will be in the wrong place. It all sounds very slack and far too casual.

Bless!

Careful Al, you’ll be stalked and emoticoned to death for that.
😀

Goodness me don simon, you making a prat of yourself on that other thread is still very much on your mind I see. Still I don’t blame you, it must have been really embarrassing for you.

And tell me, are you going to go crying to the mods now, ’cause I called a prat – like you did when I called you a dickhead ? 😀

If you two girls could put your handbags down for a minute:

http://en.wikipedia.org/wiki/Geodetic_system#Vertical_datum

Ordnance Datum Newlyn is based on Mean Sea Level at Newlyn, Cornwall between 1915 and 1921

Cornwall between 1915 and 1921

So it doesn’t even represent the current sea level anymore ? I can’t believe the casual lack of attention to detail.

I’m going remember all this next time I’m working off a datum line and I start worrying about 2 or 3 mills.

next time I’m working off a datum line and I start worrying about 2 or 3 mills.

dead-eye-dick

See the thickness of that lead Stoner ? …..that represents about one year’s worth of sea level rise that does.

So next time a site agent pulls me up saying that the heads of me door linings are a mill or two adrift from the datum line, I’ll say “give it a year or two mate, and those heads will be spot bollock correct” (I normally like to work within the “half a gnat’s bollock” system of measurement)

Layman’s question: if they carve these reference points into buildings, then how do they know that the building hasn’t subsided since the point was carved?

(I’m guessing most of this stuff is done by GPS etc these days anyways).

Caution is always advised when using old benchmarks. Most is done with GPS now but existing benchmarks are useful to double check you’ve not made a gross error…