Syrveying Using Total Station

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NIO/TR - 4 /2007
A METHOD OF TRANSFERRING G.T.S. BENCHMARK VALUE TO SURVEY AREA USING ELECTRONIC TOTAL STATION

GANESAN, P.

DECEMBER 2007

NATIONAL INSTITUTE OF OCEANOGRAPHY
REGIONAL CENTRE, VISAKHAPATNAM. INDIA

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TABLE OF CONTENTS

ABSTRACT INTRODUCTION EQUIPMENT USED METHOD OF SURVEY PERMISSIBLE ERROR LIMITS PRECAUTIONS TAKEN RESULTS AND CONCLUSION ACKNOWLEDGEMENT REFERENCES LIST OF TABLES AND FIGURES

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A METHOD OF TRANSFERRING G.T.S. BENCHMARK VALUE TO SURVEY AREA USING ELECTRONIC TOTAL STATION GANESAN, P. ( National Inst. Of Oceanography, Regional Centre, Visakhapatnam. ) (Email: ganeshan@nio.org Phone:0891-2539180) (Keywords: G.T.S. Benchmark, Mean sea level, Chart datum. Tide pole, Inter-tidal zone)

ABSTRACT
A G.T.S. (Great Trigonometrical Survey) benchmark is a permanently fixed reference survey station (or point), having known elevation with respect to a standard datum (mean sea level). These are established all over India by Survey of India department with greater precision. A benchmark value is quite essential at any survey area, especially for reduction of observed sea level with respect to mean sea level or chart datum (CD). While carrying out bathymetric survey of a survey area, the datum referenced values thus obtained are used to compute the final depth contours of the survey area (with respect to CD). So, a benchmark, having known elevation is quite essential at the survey area, without which, preparation of a bathymetric chart is impossible. In some places, GTS benchmarks are available within a kilometer distance and can be easily transferred to the survey area by fly leveling using an automatic Level instrument and a graduated leveling staff. But in most of the cases, GTS benchmarks may be at far away distance from the area to be surveyed. In these cases, the most common traditional method of transferring the benchmark value using an automatic level instrument is a difficult task, consuming enormous amount of time and labour. To eliminate this process, a method is suggested in this technical report to transfer GTS benchmark from any far distance to the survey area. A latest Electronic Total Station (ETS) is the instrument that can be used for this purpose. The main advantage in applying this method is that considerable amount of time can be saved while maintaining the required accuracy.

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INTRODUCTION
A benchmark (B.M.) is defined as a fixed reference point of known elevation with respect to a standard datum. A datum (or plane) is any arbitrarily assumed surface level (or line) from which vertical distances are measured. G.T.S. benchmarks are the ones, which are established at spacial intervals all over the country, with very high precision by Survey of India (SOI) department. Their geographical position and elevation above the standard datum (mean sea level) are given in catalogues, as well as, on G.T.S. maps, published by SOI. While carrying out bathymetric surveys or observation of tides, these benchmarks are very much essential to refer the readings with respect to mean sea level or chart datum. If these G.T.S. benchmarks are available near the survey area, they can be easily used for this purpose. However, if these benchmarks are located beyond 5 kilometers away from the survey area, transferring this level to the survey area becomes very tedious, especially using a combination of an automatic level instrument (Fig: 1) and a graduated leveling staff (Fig: 2), which has been the traditional method so far. The traditional method being very tedious and time consuming, a better method is being suggested in this report.

ELECTRONIC TOTAL STATION
In the improved method, to obtain the geographical positions of baseline points “A” and “B”, (Fig: 1), “Ceeducer” differential global positioning system (Fig: 4) can be used. This instrument computes geographical position of any point using the raw data collected from four satellites in the horizon. Also, it receives transit...
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