What is a Benchmark in Surveying?
Benchmark is a permanent and temporary reference point in surveying. The term is usually applied to any item used to mark a point as an elevation reference.
Type of Benchmark in Surveying?
It is a relatively permanent point of reference whose elevation for some assumed datum is known. It is a starting and ending point in levelling. The following four types of bench-marks are commonly used
- GTS Benchmark in Surveying
- Permanent Benchmark in Surveying
- Arbitrary Benchmark in Surveying
- TBM in Surveying (Temporary Bench Mark)
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GTS Benchmark in Surveying
A GTS Benchmark full name Great Trigonometrical Survey. GTS is permanently fixed reference survey station, having known elevation related to respect related (MSL = Mean Sea Level). All these are established all over India by a survey of India department with greater precision.
A benchmark value is quite essential in any survey area, especially for the reduction of sea level concerning mean sea level or CD (CD = Chart Datum).
While carrying out a bathymetric survey of a survey area, the datum referenced values so obtained are utilized to compute the final depth contours of this survey area for CD.
Thus, a benchmark, having known elevation, is quite essential in the survey area; without that, preparation of a bathymetric chart is impossible.
In some places, GTS benchmarks are available within a kilometer distance and can be easily moved to the survey area by fly leveling using an automatic Level instrument along with a graduated leveling staff.
However, in the majority of the cases, GTS benchmarks might be at much away distance in the area to be surveyed. In such cases, the most common traditional way of transferring the benchmark value using an automatic level instrument is a difficult job; consuming an enormous amount of time and labor.
To eliminate this process, a method is suggested in this technical report to move the GTS benchmark from any distance to the survey area. The latest Digital Total Station (ETS) is a tool which may be used for this purpose.
The main advantage of applying this method is a considerable amount of time could be saved while maintaining the required accuracy.
The long-form of the GTS benchmark is the great Trigonometric Survey benchmark. They’re established by national agencies like a survey of India. They’re established with the highest accuracy at several locations all around the country, depending on the mean sea level.
The survey of India has used a mean sea level of Karachi (Pakistan) as a datum. All these benchmarks are indicated on a brass plate fixed to a concrete pedestal with well-protected wall.
The U.K. Ordnance Survey and the U.S. Geological Survey have established similar, standard benchmarks in their countries. These benchmarks are used as reference or starting points for all important surveying work.
As Per Below list agencies for different countries.
- India = Survey of India
- United Kindom = Ordnance Survey
- Pakistan = Survey of Pakistan
- New Zealand = Land Information New Zealand
- Canada = Natural Resources Canada
- Japan = Geographical Survey institute
- United States = The United States Geological Survey
- France = Institut Géographique National
- Republic of Ireland = Ordnance Survey Ireland
- Italy = Istituto Geografico Militare
- South Africa = Department of Rural Development and Land Reform
- Spain = Instituto Geográfico Nacional (IGN)
GTS Benchmark in US
GTS Benchmark in India
GTS Benchmark in United Kindom
GTS Benchmark in Pakistan
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Permanent Benchmark in Surveying
Permanent benchmarks are fixed reference points established by government agencies like the erstwhile Public Works Departments in certain states, starting from the standard benchmarks such as GTS.
Conspicuous points onto culverts. Bridges and buildings are normally chosen. The exact position of the benchmark might be marked with a small rectangle or arrow, as shown in Fig. No. A bronze tablet might also be utilized.
Permanent Benchmark setup by state Government agencies like the Public Work Department (PWD) fix such a benchmark.
Arbitrary Benchmark in Surveying
Arbitrary Benchmarks In most engineering projects, the difference in elevation is much more important compared to a reduced level when it comes to mean sea level.
In these cases, the elevation of a permanent structure such as a corner of the plinth of a building might be assumed to have an arbitrary reduced level, like 100.00 m or 500.00 m. All these benchmarks are quite useful in small projects.
TBM in Surveying (Temporary Bench Mark)
Temporary Bench Mark
This type of benchmark (TBM) is established at the end of the day work,
Next day work might be continued from there.
Such point ought to be to a permanent object to ensure next day It’s easily identified.