An ordnance datum (OD) is a vertical datum used by an ordnance survey as the basis for deriving altitudes on maps. A spot height may be expressed as above ordnance datum (AOD). Usually mean sea level (MSL) at a particular place is used for the datum.

Global Positioning System (GPS) and the OSGM15 TM model (the height transformation between the European Terrestrial Reference System 1989 and the national height datums) is the preferred method of heighting used by Ordnance Survey. This method achieves the most accurate and uniform method of heighting.

TBMs may be surveyed in to the OS Datum by levelling between the site TBMs and an OSBM. A site datum may be established instead and all levels referred to a TBM that has been given an arbitrary value (usually 100.000 metres, or a value that ensures all heights will be positive). TBMs require to be stable.

In levelling operations, a level line is chosen to which the elevation of all points is related and is known as a datum or datum surface. This can be any surface but the most commonly used datum is mean sea level and, for Great Britain, this is the mean sea level as …

Ordnance Datum and any heights referred to Ordnance Datum are said be Above Ordnance Datum (AOD). All heights marked on OS maps and plans will be AOD. On many construction and civil engineering sites, mean sea level is not often used as a datum for levelling. Instead, permanent feature of some

The datum for mainland Great Britain is mean sea level at Newlyn; the Ordnance Datum (OD). The first primary levelling across England, Wales and Scotland, was carried out 1841-60. The first datum was an arbitrary height 100 feet below a bench mark cut in the face of the tower of St John’s Church, Old Haymarket, London; in 1844 a datum at ...

2018年5月1日 · In these regions the BMs cannot be relied upon to accurately define the ODN (Ordnance Datum Newlyn – mean sea level determined at Newlyn in Cornwall). Today, access to the national height reference is through the OS Net permanent GPS stations (this post explains the technicalities in more detail).

2007年7月11日 · To my knowledge, ASME Y14.5M-1994 says that a datum feature symbol is [A] with a 60deg triangle for a "arrowhead". I am unsure of the ISO equivalent for GD&T. So to sum it up, is it best practice to include dashes before and after datum letter, or not to?

The mathematical relationships between different spheroids are expressed in the form of so-called datum-transformations (or simply ‘datum-shifts’). Figure 2. Geocentric and Geographic Coordinate Systems.

Datums and datum systems are the tools you use to constrain a tolerance zone for one feature on the workpiece to one or more other features.