Land Survey

There are numerous land survey techniques that are used in the various situations that we mentioned below, however there are five fundamental techniques that are used most commonly among land surveyors: triangulation, trilateration, traverse, leveling and radiation.

Triangulation land survey technique uses series' of connected triangles that join and overlap each other, and from there angles can be measured from determined stations. This is the most commonly used land survey technique and is also very efficient as it minimizes the number of measurements that need to be made.

Trilateration is a land survey technique that uses electronic distance measuring equipment to measure the lengths and sides of the triangles used in triangulation and from there the angles can be calculated. Trilateration is a preferred land survey technique in rough terrain where it can be easier to get accurate calculations than by using the traditional triangulation land survey technique.

Traverse land survey technique uses a series of lines whose distances and lengths have been measured and are connected together by points in determined locations. Traverse lines can be either open or closed and can be adjusted to go around rough terrain or obstacles as required. This land survey technique is commonly used for creating preliminary surveys for the building of new roads.

Leveling is a land survey technique that is used to determine the difference in elevation in a specified area by measuring vertical distances on a graduated rod using a leveling instrument. There are a variety of leveling instruments, however dumpy levels, transits and Theodolites are the preferred option. Using trigonometry the difference in elevation between two points can be established.

The final land survey technique is Radiation. Radiation is a land survey technique that is used most commonly in conjunction with a plane table. Using a fixed position above a ground location various points are taken at the boundary of the survey area. These points are drawn on a piece of paper and the distances measured and converted to the required scale on the survey sheet. Radiation is commonly used with other land survey techniques such as traverse and triangulation.

Cadastral Survey Such a survey is done either to determine the lengths and directions of boundary lines and the area bounded by these lines, or to establish the positions of these boundaries. This is termed as Cadastral Survey.

Engineering Survey (Topographical survey, As-built surveys)
The purpose of such a survey is to determine the configuration of the ground (Topographical Survey), a survey made to locate or lay out engineering works are known as Engineering Surveys (Construction Surveys).

Gas-pipe and Power cable surveys
This survey is done to lay out the position and the length of the gas-pipe(s) or power cables and after the pipe or cable is laid, the survey is carried out to determine its final laid position and length.

Tunneling Survey
This is a survey conducted to locate and/or lay out a tunnel's position. This is a rather specialized field with the survey calculations aiding to guide a Tunnel Boring Machine in the correct direction and position.

Deformation Monitoring Survey
This survey is carried out to determine the effects, if any, on an influenced zone (usually an area in or surrounding construction works). It is a repetitive survey conducted at regular intervals such that the data collected over time can then be analyzed for a trend to determine any effects. This kind of survey is further categorized into 2D or 3D monitoring.

A 2D example being Ground Settlement Monitoring while 3D monitoring is typically employed for structures. Finally, because of its repetitive nature, in certain areas (such as tunnels) it is performed automatically vis-à-vis installing an automated monitoring system.

Automatic Tunnel Monitoring System
ATMS is short for Automatic Tunnel Monitoring System. It is categorized under Deformation Monitoring Survey and is a fully automated system. It will run 24/7, 365 days a year, continuously without any need for human intervention once the system is commissioned and initialized. It is fully capable of measuring the absolute displacement and distortion of the tunnel.

As described under Deformation Monitoring Survey, deformation could possibly be accelerated when there are on-going construction activities nearby. Therefore, to safeguard existing structures and prevent any unforeseen circumstances from taking place, continuous monitoring of the structure(s)' stability is of paramount importance.

Bearing provides automated tunnel deformation monitoring in a survey context. Succinctly, an Automatic Tunnel Monitoring System (ATMS) is set up in the tunnel to carry out monitoring activities regularly throughout the day, throughout the period of contract. It is further complemented by intermittent manual checks to ensure results integrity.

Global Positioning System (GPS) Technique
The Global Positioning System (GPS) is a network of about 30 satellites orbiting the Earth at an altitude of 20,000 km. The system was originally developed by the US government for military navigation but now anyone with a GPS device, be it a SatNav, mobile phone or handheld GPS unit, can receive the radio signals that the satellites broadcast.

Wherever you are on the planet, at least four GPS satellites are 'visible' at any time. Each one transmits information about its position and the current time at regular intervals. These signals, travelling at the speed of light, are intercepted by your GPS receiver, which calculates how far away each satellite is based on how long it took for the messages to arrive.

Once it has information on how far away at least three satellites are, your GPS receiver can pinpoint your location using a process called trilateration.

Imagine you are standing somewhere on Earth with three satellites in the sky above you. If you know how far away you are from satellite A, then you know you must be located somewhere on the red circle. If you do the same for satellites B and C, you can work out your location by seeing where the three circles intersect. This is just what your GPS receiver does, although it uses overlapping spheres rather than circles. The more satellites there are above the horizon the more accurately your GPS unit can determine where you are.

GPS and Relativity
GPS satellites have atomic clocks on board to keep accurate time. General and Special Relativity however predict that differences will appear between these clocks and an identical clock on Earth. General Relativity predicts that time will appear to run slower under stronger gravitational pull – the clocks on board the satellites will therefore seem to run faster than a clock on Earth. Furthermore, Special Relativity predicts that because the satellites' clocks are moving relative to a clock on Earth, they will appear to run slower.

The whole GPS network has to make allowances for these effects – proof that Relativity has a real impact.

As briefly covered in the earlier section, BEARING has modern equipment and skilled manpower to carry out hydrographic surveys for various applications. The equipment used by BEARING for such surveys, CEEDUCER Pro, is a light and portable equipment, and can be used for long durations without extensive power requirements. The equipment stores latitude, longitude and depth data in internal memory and is completely splash proof, making it ideal for working with small boats in water bodies like rivers, reservoirs and lakes.

The equipment has an inbuilt GPS. Depth data is available 6 times every second.

There are various transducers available, and 210Hz is generally used for shallow water surveys.

Depth to RL conversion requires water level data, which is measured on hourly basis at gauge locations.

Ceeducer has an inbuilt temperature correction. Bar checks are carried out twice a day to calibrate the readings as speed also changes due to salinity etc.