Deformation monitoring which is also known as deformation survey

Deformation monitoring which is also known as deformation survey, is a type of measurement and tracking of the dimensions of any object. Deformation monitoring is an important concept of measuring values that can be used for computation, deformation analysis, building maintenance. Most of the time, deformation monitoring is related to the field of surveying, but may also related to civil engineering, mechanical engineering and construction.
Deformation can also refer to the changes of an objects in its shapes, dimensions and position. Big engineering structures or monuments need deformation monitoring due to the factors such as natural disasters, land movements and tectonic phenomena. Therefore, it is important to measure the objects movements for the purpose of safety assessment and preventing any disaster in the future (Wan Mohd Akib, Kok, & Zulkarnaini Mat Amin, 2004)
The methods or measuring techniques can be sorted in two main groups, geodetic and geotechnical measurements. Both measuring techniques can be widely used in this era of modern deformation monitoring.
i. Geodetic measuring devices measure georeferenced (relative to established locations outside the monitoring area) displacements or movements in one, two or three dimensions. The examples of equipment used are total stations, digital level, and global navigation satellite system receivers.
ii. Geotechnical measuring devices measure the displacements and the related environmental conditions without external dereferencing. It includes the use of instruments such as extensometers, piezometers, pressure meters, rain gauges, thermometers, barometers, tilt meters, accelerometers, seismometers etc.
Each measurement type has its own advantages and disadvantages. Geodetic surveys, through a network of points interconnected by angle and distance measurements, usually supply a sufficient redundancy of observations for the statistical evaluation of their quality and for a detection of errors. Geodetic measuring gives global information on the behavior of the structure while the geotechnical measurements give very localized and locally disturbed information without any check unless compared with some other independent measurements. On the other hand, geotechnical instruments are easier to adapt for automatic and continuous monitoring than conventional geodetic instruments. Geodetic surveys are labor intensive and require skillful surveyors, while geotechnical instruments, once installed, require only infrequent checks on their performance. Geodetic surveys have traditionally been used mainly for determining the absolute displacements of selected points on the surface of the object with respect to some reference points that are assumed to be stable. Geotechnical measurements have mainly been used for relative deformation measurements. However, with the advance of technological production, of the last few years, the differences between the two techniques are not as obvious as twenty years ago.
Meanwhile, there are many problems when using conventional monitoring methods and GPS can solve these problems very well. The global positioning system is a satellite-based navigation system consisting of a network of 24 orbiting satellites that are located at an approximately a thousand nautical miles away in space and in six different orbital paths. The satellites will continuously move, making two complete circles around the Earth in just under 24 hours.
The GPS signal contains 3 types of data, a ‘pseudo-random code’, ephemeris data and also an almanac data. The pseudo-random code will identify which satellite is transmitting the data. Then, the ephemeris data is constantly transmitted by each satellite which contains important information such as status of the satellite, current date and time. Without this part of the message, the GPS receiver would have no idea on what will the current time and date be. A specific position will be determined by using this part of signal. Next, the almanac data will tell the GPS receiver where each GPS satellite should be at. Each satellite that transmits almanac data shows the orbital information for that satellite and for every other satellite in the system (Appleseed, 2018).
In deformation monitoring, compared with the traditional monitoring methods, using GPS not only just give the advantages of high precision, fast speed, simple operation and so on, but also achieve automation purpose from data collecting, transmitting, management to the deformation analysis and prediction and real-time monitoring of remote online network by using computer technology and data communication technology. GPS monitoring data collection can be used in automatic mode and it does not need people to intervene. Monitoring data can be transmitted to the control center through the network in real-time, user can easily get real-time monitoring data from the control center, facilitate analysis of deformation after data processing. The user has no need to stay long-time in GPS receiver side, as long as can provide continuous power supply power. In order to achieve data collecting, transmitting, processing, analyzing and alarming automation. GPS can achieve long-term continuous monitoring, reduce monitoring costs. GPS monitoring has high precision. Setting fixed permanent observation posts on the roof top of the building, can reduce the centering error, leveling-up error, antenna height error and so on during monitoring in high rise building.
In many monitoring applications, the GPS technique offers significant advantages over other measurement techniques. GPS allows a high rate of measurement, over long distances between the control and monitoring points and does not require line of sight to the control points.
The advantages of GPS techniques compared to conventional deformation monitoring techniques are:
1. GPS requires no line-of-sight between the stations
2. Automated operation with high observation rate
3. Real-time data updates
4. Operates under all weathers condition.
However, the accuracy of a GPS based system is limited by the satellite geometry and by systematic errors such as multipath or weak satellite geometry. The concept of Network-DGPS was introduced to overcome the constraint of the short baseline limitation of conventional RTK technique. The Network-DGPS also will expand the coverage area of GPS reference station infrastructure (Khoo, Tor, & Ong, 2010).