points above sea level

[bronners]

how do they measure the points above sea level.the reason i ask is we have a marker on our cottage that somebody comes and checks every year or 2.but how the heck do they know that we are 360m above sea level?

[buildersmate]

Historically it was done using trig points, nowadays by tracking from the air.

[Jugglering]

*Sea Level* datum is taken from a known level point in Newlyn, Cornwall and is known as 0m AOD (Above Ordnance Datum). Surveys are conducted (as you say) to establish the difference in level between that point and your marker point. As the others have said, there are loads of *trig* (trignometry) points around the country from which the marker levels can be calculated. Ever seen folk with yellow/orange/red instruments on tripods around the place ? Those are the instruments that used to be used - levels, theodolites and *total stations*, although these days, satellite navigation tends to be used - some of which are accurate to a few millimetres!

[bronners]

thanks for your answes very informative

[sddsddean]

Actually they use bench marks. Although there are loads of trig points about, these are for Northing/Easting information. Height is done via bench marks (although a secondary bench marks are attached to trig point pillars). As said, Mean Sea Level is determined to originate at Newlyn in Cornwall. From this point, all bench marks are derived. Fundamental bench marks are usually set into bed rock and sometimes buried so they can't be interfered with. Here are some photos of FBM's.
http://images.google.co.uk/imgres?imgurl=http: //www.trigpointinguk.com/photos/P08309.jpg&img refurl=http://www.watertower.co.uk/Levelling/F BM/FBM.html&h=409&w=614&sz=36&hl=en&start=6&tb nid=OME1Ed4VRUKXhM:&tbnh=91&tbnw=136&prev=/ima ges%3Fq%3Dbench%2Bmark%2Bordnance%2Bsurvey%26g bv%3D2%26svnum%3D10%26hl%3Den%26sa%3DG%26ie%3D UTF-8
Secondary bench marks are a brass plate with holes for locating bracket to set your levelling staff on.
http://images.google.co.uk/imgres?imgurl=http: //mapzone.ordnancesurvey.co.uk/images/didyoukn ow/flushbracket.jpg&imgrefurl=http://mapzone.o rdnancesurvey.co.uk/mapzone/didyouknow/whatis/ q_12_79.html&h=250&w=250&sz=37&hl=en&start=6&t bnid=Yfe2LXqdPbyR-M:

[sddsddean]

Tertiary bench marks are the most common and are cut into the brickwork/stone of buildings and monuments.
http://www.carlscam.com/warmem/barnston3.jpg
With the demolition of so many old buildings, it is becoming harder to find bench marks that are shown on maps. Often you find the building has gone!