China
Africa
Explore chapters and articles related to this topic
Maps, drawings and surveying
Published in Malcolm Copson, Peter Kendrick, Steve Beresford, Roadwork, 2019
Malcolm Copson, Peter Kendrick, Steve Beresford
The term ‘levelling’ is used to describe the operation of finding the difference in height between points on the earth’s surface. These heights are given relative to a plane or datum which is known as the Ordnance Datum (OD). This is the mean level of the sea and is measured at Newlyn, Cornwall. Levelling is only accurate over relatively short distances as the line of sight is a tangent to the earth’s surface and longer sights would need to be corrected to take account of the earth’s curvature as in Figure 2.45.
A
Published in Les Goring, Residential Surveying Matters and Building Terminology, 2023
Levelling terms used in site-levelling: See Figure 22 on page 56: There are a number of tripod-mounted levelling instruments available nowadays, but the common principle of their use is that they provide a theoretical horizontal line (plane), encompassing the whole building site when the head of the level is pivoted around a 360⁰ circle. In levelling terms, this theoretical plane is referred to as the height of collimation, or the collimation-height, from which a variety of levels can be measured and/or established. These levels are taken (or established) from a measuring rod known as a levelling staff. On large building sites, the terms used in levelling are: Collimation height: The height of a level’s theoretical viewing plane above the original Ordnance datum established at Newlyn in Cornwall.Backsight (B.S.): The first staff-reading taken from the collimation-height above the site datum/OBM (Ordnance Bench Mark).Datum: A solid and reliable fixed point of initial reference, such as an OBM or a nearby manhole (inspection chamber) cover in the road.Foresight (F.S.): The last staff-reading before a levelling instrument is repositioned.Intermediate sight (I.S.): All readings taken other than Backsight and Foresight.Ordnance Bench Mark (OBM): A reference point (above ground), set originally by the National Ordnance Survey Department, as being a measured height above the mean sea-level at Newlyn in Cornwall, UK., at a certain time and date.Temporary Bench Mark (TBM): This refers to a temporary datum transferred from the OBM and set up on site at a reduced level – or to the same level as any alternative datum used, such as a manhole (inspection chamber) cover in the road – from which the various site levels are set up more conveniently. The on-site datum is usually in the form of a datum peg, of either a 19mm/¾ in. diameter steel bar, or a 50 x 50mm/2 x 2 in. wooden stake, set in the ground, usually partly-encased in concrete.Reduced Level (R.L.): Any calculated level position above the original Ordnance datum.
Mining large-gradient subsidence monitoring using D-InSAR optimized by GNSS
Published in The Imaging Science Journal, 2021
Haodi Fan, Xugang Lian, Wenfu Yang, Linlin Ge, Haifeng Hu, Zheyuan Du
The conventional monitoring method is to measure the subsidence by means of angle measurement, edge measurement, levelling and other technologies. It has the following advantages: providing the overall deformation state of the deformation body, checking the results in the form of an observation network, and great flexibility [2]. In practical applications, elevation measurement is generally obtained by geometric levelling or trigonometric levelling. Angle measurement is mainly obtained by theodolite and total station. Since the 1970s, various new types of precision photoelectric rangefinders have been widely used in deformation monitoring, but long-distance ranging needs to solve the problem of atmospheric refraction. After the 1990s, due to the wide application of GNSS, long-distance rangefinders were replaced by GNSS [7]. Compared with traditional surveying and mapping operations and methods GNSS has excellent performance and wide adaptability. On a local scale, China has used GNSS technology to monitor the deformation of a large number of mining areas and achieved a series of results [8–10]. However, these methods have obvious disadvantages: small monitoring range, long time span, low work efficiency, and large manpower and material consumption [11]. Therefore, with the progress of science, deformation monitoring technologies such as UAV remote sensing [12–14] and InSAR [15–17] have gradually emerged.
Non-contact structural health monitoring of a cable-stayed bridge: case study
Published in Structure and Infrastructure Engineering, 2019
Mehrisadat Makki Alamdari, Linlin Ge, Kamyar Kildashti, Yincai Zhou, Bruce Harvey, Zheyuan Du
Digital levelling is a surveying technique to measure height differences between points precisely. Usually, surveyors use a combination of a digital level and two levelling coded staves to determine height changes from one point to the other as illustrated in Figure 6.