Geodetic infrastructure in India
The Great Trigonometrical Survey of India was completed in 19th century under leadership of the great surveyors- Lambton and Sir George Everest. It is inconsistent and inadequate. Accuracy of the network is only of the 1st order or less. First order was defined as better than only 1 in 50,000 only. Reference surface and Datum- The Everest Spheroid was given by Sir George Everest in 1830. Center of Everest Spheroid is about a km away from the center of gravity of the Earth; hence it is non-geocentric. Thus it is inaccurate and unsuitable under present circumstances. Leveling network of India has inconsistencies. Gravity observations were not carried out and not taken in to consideration. It was not appropriately adjusted. Indian Absolute Gravity Datum does not exist. Absolute gravimeters have not yet been used to define Gravity Datum in India. Topographical maps are on Polyconic projection. Assumptions and approximations accepted make it a non-projection. The earth is assumed to be fiat and there are no distortions of any kind. The projection has created problems in digitization, compilation and integration of maps. Design of the Grid adopted in India is not satisfactory. Distortion at central parallel is 1 in 824, which is quite high. There is archaic Restriction policy, which is not transparent and hinders research and development India has to make a choice between chaos and development. These problems have been discussed in detail in this paper. How India should go about to establish new geodetic infrastructure for systematic development and research, has been described in this paper.
Geodetic Infrastructure in India is inadequate. It needs a fresh look It needs a fresh look and complete revision. Indian Geodetic Datum, The Reference Surface, Horizontal Control, The Vertical Datum, Height Control, Indian Geoid and Mean sea level, The Gravity datum, Projection System for Maps, and Indian Grid, are all inadequate or inappropriate.
Indian Geodetic Datum is based on Everest Spheroid as Reference Surface and Kalyanpur in Central India as initial point. Center of this reference surface is estimated to be about 1 km away from the center of gravity of the Earth. The datum is thus a local datum and in error. Scientific and Defense studies of vital National importance cannot be based on such a system. It is therefore extremely necessary that the Indian Geodetic Datum should be redefined at the earliest. The project on Redefinition of Indian Geodetic Datum should be taken up in right earnest and completed within one year. Horizontal Control in India resulted from The Great Trigonometrical Survey. The different triangulation series are inconsistent between each other. It has not been properly adjusted. Stations are burdened with varying degrees of error and many points are either destroyed or in need of repair. It is therefore urgently necessary that fresh observations be carried out to get a set of vectors by GPS and other means and least square adjustment be carried for the whole country at one go to get control points of zero, 1st, and 2nd order in 2 to 3 years. Vertical Datum for Heights in India was chosen as the Mean Sea Level at a group of nine tidal observatories situated at Indian ports. Level network in India is of moderate to high precision at different places. No gravity observations were carried out at that time. Network has not been properly adjusted. It is required that a fresh datum for required that a fresh datum for heights based on Mean Sea level at one tidal observatory say Mumbai. High Precision Leveling should be carried out afresh throughout India to get a network along with gravity observations and adjusted so as to have 1st order vertical control. It is necessary to have a Gravity Datum in India. Presently there is no absolute gravity station in India. Gravity values in India are based on relative gravity observations based on gravity datum/ s in other countries. It is therefore necessary to establish some absolute gravity stations, adopt a gravity datum and carry out relative gravity observations in order to get a 1st order gravity network. No satisfactory Indian Geoid is available. To get heights above Mean sea level by GPS observations we need a geoid that can give geoidal undulation of accuracy of 25 cm or better. A project should be taken up immediately to obtain geoid by gravimetric as well as astro-geodetic methods. Projection for topographical maps in India is Polyconic. Each individual sheet is projected individually assuming no distortion along parallels and neglecting distortion along meridians. Meridians and parallels are assumed as straight lines. These assumptions make it a perfect projection, which is not possible. We can say that The Earth/Ellipsoid has been assumed to be ?at for individual sheets. This creates problems in digitization, integration and compilation of maps. It is therefore necessary to abandon so called polyconic projection and adopt either Lambert Conformal Conic or Transverse Mercator Projection designing suitable zones. Indian Grid on Lambert Conformal Conic Projection for superimposition on topographical maps was designed during British days. It has 9 zones with scale factors at central parallel as 649/650 and 823/824. The designis unsatisfactory as scale error at central parallel should be 1 in 2000 or better. The grid has been restricted which is irrational as the grid parameters are available to everyone outside India including Pakistan. It is therefore necessary to design grids on Transverse Mercator or Lambert Conformal conic Projection with suitable zones similar to State Plane Coordinate systems in USA. It is suggested that each Indian state should have a grid for that state and all mapping is carried out on such grid for civil use. Structure of Monuments (geodetic stations, bench marks etc.) is not of permanent nature in many cases and many have been destroyed. Monuments that are not fixed to bedrock are not suitable for geodetic monitoring of crustal movements. New and suitable permanent monuments need to be constructed before any new project regarding geodetic infrastructure is taken up.
Indian Geodetic Datum
Indian Geodetic Datum is based on Everest Spheroid as reference surface given by Sir George Everest, then Surveyor General of India in 1830. Kalyanpur in central India was chosen as initial point or origin. Coordinates of initial point and azimuth of a
line were obtained by astronomical observations and leveling. The reference surface was however defined peace-meal at various times. Astronomical observations were carried out at least twice. More precise observations carried out later were accepted. Hence meridional and prime vertical de?ection of vertical, were de?ned at Kalyanpur. Parameters of the datum are given in table1.
Value of Semi major axis and semi minor axis were given in feet as 20,922,931.80 feet and 20,853,374.58 feet respectively. These values were converted into metres using different conversion factors resulting in many values of a and b of Everest spheroid. The of?cial conversion factor for India is 0.3047996 and should be accepted.
It is estimated that the center of Everest spheroid is about 1 km away from the center of gravity of the earth; hence it is not a geocentric datum. We therefore conclude that it is inaccurate system and needs rede?nition. The present datum is especially not suited for many geodetic, geodynamic, geophysical and defence applications. One has either to use WGS84 for Geodetic monitoring of crustal movements, plate tectonic movements, development and deployment of missiles and many other scientific applications or redefine Indian Geodetic System/Datum. A project to redefine Indian Geodetic Datum was taken up in 1989 but nothing much appears to have been done so far. Redefinition project should be taken up and given highest priority. It should be time bound and completed in 2 to 3 years. In USA a similar project was taken up in 1974 by National Geodetic Survey and completed in 1983. It is known as NAD 83 and is used for all applications including Geodynamic and defense.
Horizontal datum in India is Indian Geodetic Datum based on Everest Spheroid. Existing horizontal control in India is the result of Great Trigonometrical Survey of India consisting of 5 blocks with 2700 stations and 10 bases. Triangulation series were started from Kalyanpur. The Indian subcontinent was divided into ?ve parts region-wise, four quadrilaterals (NW, NE, SW, SE) and the Southern Trigon. The quadrilaterals could not be adjusted together due to computational limitations at that time. Several corrections viz. for defiection of vertical, skew normal and geodesic also could not be applied. In 1937-38 an attempt was made to readjust the triangulation network but this also suffered from the same limitations. Though densi?cation of control and ?lling of gaps has been done in addition to observation of more bases and Laplace stations, no fresh adjustment has been carried out. This has resulted in the various series being inconsistent with each other. The horizontal control is therefore burdened with varying degrees of errors; say from a few metres to as much as 100 metres at places. Many stations are however supposed to of 1st order that is 1 in 50,000. Most of the stations of this control are on hills covered by jungles. Many stations have been destroyed and many others in poor condition, hence not suitable for geodynamic studies and zero/1st order geodetic horizontal control.
The need therefore is to provide complete horizontal control of zero and 1st order afresh and adjust it by least squares for the whole country at one go using available scienti?c adjustment software. BIGADJUST, the software used by National Geodetic Survey of USA has been obtained by Survey of India to adjust the present control but the same has not been completed and it is not known as to what are there plans regarding this.
It is suggested that in addition to rede?nition of Indian Geodetic Datum a project should be planned to provide horizontal control of zero, 1st and 2nd order throughout India. The following steps are suggested: –
1) Identify places for monuments. Care should be taken to choose places suitable for geodynamic studies also. Rooftops of permanent public buildings can also be chosen in preference to hilltops in many cases, as the control will now be provided using GPS.
2) Design suitable monuments and carry out construction of monument pillars. It should be seen that pillars are fixed to bedrocks to be suitable for future geodynamic studies.
3) Design network and observe all vectors using dual frequency geodetic GPS receivers in relative positioning mode.
4) Process the data using a scientific software such as Bernese.
5) Adjust the data by least squares using a network adjustment software such as BIGADJUST.
6) Compile the data in a suitable format for use for various purposes and for dissemination to public.
Vertical Datum and Height Control
In India, the vertical datum for heights has been chosen as the mean sea level at a group of nine tidal observatories situated at various Indian ports. Hourly tidal observations were carried out at these ports for a number of years and averages obtained.
It was assumed that the mean sea level at these ports, belong to the same sea level surface. All these ports served as issue points for the first level net of India. Leveling net in India consists of first level net of moderate precision covering 18,000 miles started in 1858, and second level net of 16,000 miles based on first level net. Second level net was adjusted on to first level net wherever necessary. We can see clearly from the above that assumptions were incorrect. Precision was moderate and adjustment was not carried out properly. Choice of vertical datum was not unique and creates confusion. Gravity observations were not carried out which is necessary for National Level Nets of high precision and 1st order accuracy. The present heights are thus in varying degrees of error and are not of present day standards. These cannot be used as basis for geodynamic studies and many geodetic and geophysical studies where 1st order vertical control is required. It is therefore suggested that a fresh vertical datum be adopted and vertical control of 1st and 2nd order be provided by spirit leveling along with gravity observations. The following steps are suggested:-
1) Select a tidal observatory where hourly tidal observations of 18.6 years cycle of successive nodes of the moon are available. Construct a few permanent benchmarks near the observatory in stable and protected area. Provide heights of these benchmarks by 1st order spirit leveling from the chart datum to the benchmarks. These benchmarks should be taken as issue points for the fresh leveling network of India. The mean sea level obtained here should be the National Mean Sea Level for India at this observatory. Tidal observatory at Mumbai port may be chosen for obtaining the mean sea level. Design the network and construct the benchmarks along the routes selected for leveling in phases. Leveling of 1st order should be carried out along-with gravity observations using relative gravimeters throughout India.
2) Carry out adjustment of the network by least squares at one go and document the heights along with description of benchmarks.
3) Construct a few permanent benchmarks near other tidal observatories also. Find mean sea level at these observatories also and provide heights of the benchmarks constructed near the observatories from the chart datums of such observatories by 1st order leveling. These will represent the local mean sea level in those areas. There will be some difference between the national mean sea level heights and local mean sea level heights. The difference can be applied to heights in that area wherever needed based on sound statistical analysis. Scientific analysis of various mean sea levels and heights may be carried out for geoidal, geodynamic and geophysical studies.
4) All the monuments constructed for zero and 1st order horizontal control, should also be connected by 1st order spirit leveling. Monuments constructed for geodynamic studies should also be similarly connected.
Presently we do not have a gravity datum in India. A 1st order gravity station exists at Palam airport Delhi, which was provided by relative gravity observations from other countries. A precise gravity network of 42 stations was established in 1971 by Survey of India covering airports of the country with an uncertainty of + or – 0.05 mgal. These stationsserved as reference for future gravity surveys in India. This gravity network was adjusted within the framework of International Gravity Standardization Net 1971 (IGSN 71). La-Coste and Romberg model G gravimeters were used for observations.
Until recently we did not have any absolute gravimeter. Recently NGRI has acquired one absolute gravimeter, which is being used by them for scientific research. We have to plan establishment of absolute gravity datum in India and connect existing gravity stations to the absolute gravity station/stations. It is hoped that NGRI will take leading part in this project. All first order horizontal control monuments and 1st order leveling bench marks should be connected to gravity network so as to have 1st order gravity values. This will facilitate precise repeat gravity observations for geodynamic applications such as
1) Detection and interpretation of vertical ground motion in earthquake
2) Monitoring and interpretation of post earthquake motion.
3) Postglacial rebound studies.
4) Monitoring of movement of magma in volcanic areas along with leveling.
5) Reservoir depletion studies of all kinds.
6) Tectonic motions and crustal warping studies.
Presently no satisfactory Indian geoid is available. To obtain heights above MSL with GPS we need a geoid, which can give geoidal undulation accuracy of about 25 to 50 cm or better. It is therefore necessary that a project should be taken up to determine Indian geoid by gravimetric as well as by astro-geodetic methods.
Projection for Maps
All topographical maps in India are on polyconic projection. Assumptions and approximations applied to it make it a mockery if we say that a projection has been adopted. The sheets on 1:25,000; 1:50,000; and1 :250,000 topographical sheets are individually projected assuming that distortion along meridians can be neglected. There is no distortion along parallels. Besides these the meridians and parallels are joined by straight lines. This means that it is not a projection. It amounts to assuming that the earth is fiat in respect of individual sheets. It has created a lot of problems in integration of different maps, compilation of maps, digitization and hence in GIS. We should change over to Lambert Conformal Conic or Transverse Mercator/UTM for our topographical maps after forming suitable zones. Individual states of India should adopt one of the two projections for all mapping in their states similar to State Plane Coordinate System in various states of USA.
Indian grid was designed during British days dividing India into 9 zones in Lambert Conformal Projection. The grid is not satisfactory as scale error at central parallels is 1in 850 and 1 in 650. Distortion is considered high. We should aim for 1 in 2500 but should not be more than 1 in 1000. Restriction of the grid is also irrational as parameters and all information about it is available to everyone anywhere in the world except in India. There is therefore an urgent need to design grids afresh. We should adopt either Lambert Conformal Conic or Transverse Mercator depending upon whether the area to be projected is greater in E-W extent or N-S extent.
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