Geodetic infrastructure in India
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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