definitions

level line

a level line or level surface is one which at point is normal to the direction of the force of gravity as defined by a freely suspended plumb - bob . As already indicated in chapter / in the discussion of geoid  

such surfaces are ellipsoidal in shape . the differnce in level between A and B is the distance A B , provided that the non-parallism of level surface is ignored . 

horizontal line :

A horizontal line or surface is one that is normal to the direction of the force of gravity at particular point . show a horizontal line through point c 

datum :

a datum is any refernce surface to which the elevations of point are . the most commonly used datum is that of mean sea level (MSL)

in the uk the MSL datum was measured and established by the ordance survey (OS) of Great Britain , and hence it is often referred to as ordnance datum (OD) it is the mean level of the sea at newlyn in Cornwall calculated from hourly reading of the sea level .

taken by an automatic tide gauge over a six - year period from 1 May 1915 to 30 April 1921 . the reading are related to the Observatory Bench Mark , which is 4.751 m above the datum . Other countries have different datumes ; for instance , Australia used 30 tidal observatories . interconnected by 200 000 km of levelling . to produce their national datum , whilstjust across the English Channel , France uses a different datum , rendering their levels incompatible with those in the uk .

horizontal and level lines

bench mark (BM) :

in order to make OD accessible to all users throughout the country , a series of permanent marks were estab lished , called bench marks . the height of these marks relative to OD has been established by differential levelling and until about 1970 was regularly checked for any change in elevation .

(1) cut bench marks 

the cut bench mark is the most common type of BM and is usually cut into the vertical surface of stable structure 

(2) flush brackets

flush brackets are metal plates 180 mm × 90 mm , cemented into the face of buildings and were established at intervals of about 2 km 

(3) bolt bench marks 

bolt bench marks are  60 - mm - diameter brass bolts est in horizontal surfaces and engraved with an arrow and the letters OSBM 

(4) fundamental bench marks (FBM)

in the UK , FBMs were established by precise geodetic leveling . at intervals of about 50 km . Each mark consists of a buried chamber containing two reference points , while the published elevation is to a brass bolt on the top of a concrete pillar 

rivet and pivot BMs are also to be found in horizontal surfaces . 

Detals of BMs within the individuals area of interest may be obtained in the form of a Bench Mark List from the OS . Their location and value are currently also shown on OS plans at the 1/2500 and 1/ 2500 scales . their values are quoted as precise to the nearest 12 mm relative to neighbouring bench marks at the time of the original observation only .

Bench marks established by individuals , other than the OS , such as engineers for constuction work are called temporary bench marks (TBM)

cut bench marks

DESCRIPTION :

these are normally placed on vertical walls of building and in the sides of triagulation pillars . they are cast in brass and rectangular in shape ( 180 mm × 90 mm ) with a large boss at the rear of the plate . the boss is cemented int a prepared cavity . so that the face of the bracket is vertical and in line with the face of the object on which it is placed . these marks in precise leveling necessitate the use of special fitting as above each flush bracket has a unique serial number and is referred to in descriptions as FI , Br , NO .........

they are sited at approximately 2 kilometre intervals along Geodetic lines of levels and at 5 to 7 kilomete intervals on secondary lines of levels .

Reduced level : (RL)

the RL of a point is its heigh above or below reference datum.

DESCRIPTION:

the sites are specially selected with reference to the geological structure , so that they may be placed on sound strata clear of areas liable to subsidence , they are established along the geodetic lines of throughout great britain at approximately 50 kilometere intervals . 

they have three reference point two of which , a gun metal bolt set in the top of a pillar projecting about i foot above ground level . the piller bolt is the reference point to be used by tertiary levellers and other users .

the buried chamber is only openedon instructions from headquarters some fundamental bench marks are enclosed by iron railing , this was done where necessary , as aprotective measure , these marks are generally referred to as F.B.Ms