DGPS IN CIVIL ENGINEERING
Written By: Avinash Pandey
ADVANCEMENT IN SURVEYING TECHNIQUES
By 2600 BC, Egyptians created the earliest surveying instruments: plumb board, A-Level, T-Level, and plumb square. The plumb bob still remains an important tool in modern construction. When it was noticed that the lodestones (a mineral composed of iron oxide) used by Chinese fortune tellers aligned itself in the North-South direction and were better at pointing out real directions, it led to the first compass. Through the 1800s, surveying was performed using a compass and a “chain”. Eventually, the use of the compass gave way to the transit and the chain to the steel tape. Now, Electronic Distance Measurement (EDM) technique is used in surveying. Theodolite and Total Station(TS) works on the EDM technique. Is it time to adopt new techniques that make surveying much easier??
WHAT IS GNSS????
The general public is so familiar with the term GPS(GLOBAL POSITIONING SYSTEM) that it has become synonymous with any satellite navigation system. But it is not the case. GPS is a result of a satellite navigation experiment conducted by the US Navy to navigate US submarines carrying nuclear missiles. As of now even some of the civilian applications have access to some of the GPS signals. There are other systems too. GLONASS of Russia and BeiDou-2 are some examples. Global Navigation Satellite System(GNSS) is the generic name used to describe any global system of satellites that transmit signals for navigation purposes on Earth. The most basic option for digital surveying is a single GNSS receiver.
DGPS
DGPS stands for Differential GPS. DGPS is like an upgrade to GNSS receivers. A local coordinate system is used in DGPS. Static positioning is a carrier phase-based relative positioning technique that employs two (or more) receivers simultaneously tracking the same satellites. In static DGPS surveying sessions, receivers are motionless on the ground throughout the observation. Static positioning is usually done to establish control points as static work often provides higher accuracy and redundancy than kinematic work. Also, the results are processed after the session is completed.
Real-Time Kinematic(RTK) is a technique to enhance the accuracy of position data measurements derived from GNSS. Like static positioning, RTK surveying is a carrier phase-based relative positioning technique except that receivers are in motion. RTK technique is used for surveying purposes.
FIG: REAL-TIME KINEMATICS
Differential GPS involves the cooperation of two receivers, one that’s stationary (BASE) and another that’s roving around making position measurements (ROVER) both attached to a radio transmitter. ROVER GPS receiver uses range corrections to remove correctable errors. Among two sets of equipments, anyone can be used as a base or rover.
FIG: BASE STATION
FIG: ROVER
FIG: ROVER GPS RECEIVERFIG: ROVER GPS RECEIVER
DGPS TECHNIQUE OVER EXISTING SURVEYING METHODOLOGY??
The most advanced method of surveying is the technique of Electronic Distance Measurement (EDM). In this method, the distance between two points is determined using electromagnetic waves. Since TS is widely used nowadays, we will compare DGPS with TS:
Surveying with a TS(TOTAL STATION), unlike DGPS, is restricted to measurements between inter-visible points. Often control points are located distant to the survey area, and traversing with TS to propagate control is time-consuming. The inter-visibility between the instrument and prism is needed in TS whereas inter-visibility is not needed in the case of DGPS. Data collection can also be done at night time while surveying with DGPS, unlike TS. Even TS requires the technique of DGPS for various purposes like knowing the coordinates of benchmarks and establishing ground control points. A drone can be used to generate contour too and works much better with DGPS.
Here are some advantages of using DGPS over TS :
a) BETTER ACCURACY
The coordinates measured by DGPS are of higher accuracy than that of TS. Measurements of accuracy across the Atlantic, in Portugal, using DGPS estimated an error of just 0.22m/100km. The distance accuracy of TS has an error range of (10 to 12)mm/km.
b) SAVES TIME
While surveying an area with TS, the base station needs to be shifted whenever there is an obstruction while collecting data. Since DGPS operates on signals received by the ROVER GPS receiver, there is no need to shift the base station when ROVER is within the range. So, there is no additional time required for the centering and leveling of the instrument every time the base station is shifted.
c) EASY DATA COLLECTION
Unlike in TS, there is no need to fix a reflector at control points whose coordinates are to be determined. Just rovering around the area to be surveyed will work in the case of DGPS while working on Auto Survey mode. The instrument will automatically record coordinates of points for every meter you rover.
FIG: ROVERING AROUND THE SURVEY AREA
FIG: ROVERING AROUND THE SURVEY AREA
DGPS technology has brought many positives, but also some downsides. Most of the errors are either eliminated completely or made negligible after using DGPS. However, there exist some. Here are some of the prominent negatives associated with the use of DGPS:
a) EXPENSIVE
DGPS is very costly. Owing to its high cost, developing countries like Nepal may find it unaffordable and are compelled to limit its usage.
b)WEATHER EFFECT
Weather is something beyond our control. DGPS needs the weather to be clear as it needs to send and receive signals for data collection. Signals get affected due to gloomy weather. So, this leads to errors in data collection.
c) NOT USEFUL IN DENSE AREAS
DGPS is not effective to use in dense areas like forests. The position error increases with the increasing density of the forest.
CONCLUSION
TS and DGPS both are used within different applications depending upon the requirement. Both the techniques are advanced methods of surveying. Comparing these techniques, DGPS is an accurate and versatile geomorphological tool. DGPS can measure points without any line of sight requirement and is highly accurate. Because of the direct acquisition of 3D coordinates, DGPS is more effective for surveying than TS in terms of accuracy and fieldwork. The time requirement and the field surveying works for point determination using DGPS is comparatively less than that of TS. DGPS is one of the best choices to collect data, however in case of signal blockage, like in dense areas, it should be aided by TS. So the user needs to decide on the method to apply depending upon the accuracy required.
1 Comments
So informative. Keep up the good work.
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