Error Estimates
With a view to work out the extent of error observed while transforming WGS-84 into India-1956 System (Everest) or vice-versa, an attempt was made to use a software which uses the average values of the parameters of Everest System Ellipsoid for mathematical conversion of India-1956 (Everest) to WGS-84 or vice-versa. For this purpose MADTRAN developed by the National Imagery and Mapping Agency (NIMA) of USA was selected for mathematically converting the coordinates from Everest to WGS –84 of certain selected points located inside the airport premises in India. Further;
- Actual GPS observations were made to get the precisely measured coordinates of those points in WGS-84 system and a comparison was made to determine the extent of error between these two observations.
- Direct comparison of the values of WGS-84 and Everest Coordinates of various points without conversion was also done to determine the maximum extent of error between these two systems of coordinates.
- These observations (and comparison) were done for Latitudes and Longitudes only.
For this purpose eleven airports geographically located towards the Northern, Southern, Eastern and Western parts of India viz., Delhi, Kolkata (earlier known as Calcutta), Mumbai (earlier known as Bombay), Ahmedabad, Coimbatore, Tiruchirrappalli (Trichy), Chennai (earlier known as Madras), Calicut, Trivandrum, Hyderabad and Bangalore were selected. Prolonged and continuous GPS observations were made at the Aerodrome Reference Points (ARPs) inside those Airports and precise WGS-84 coordinates were measured. The geo-coordinates of the above mentioned airports were also transformed from Everest to WGS-84 by using MADTRAN software. The observation results are summarised and presented in Table 1.
It may be mentioned that the Aerodrome Reference Point (ARP) is taken as the centre of the airport and it is physically located at a suitable place as close to the middle of the airport as possible. The ARP is treated as the standard benchmark and the geographical coordinates (latitude, longitude and elevation) of the ARP is precisely determined in Everest Coordinate System by some expert agency like Survey of India. The remaining areas of the airport are then surveyed by the survey team of Airports Authority of India (AAI) by taking the ARP as the starting point, and the coordinates and elevations of other important points of the airport are connected to the ARP. In view of the above, it was decided to take the GPS observation at the ARPs of the above mentioned 11 airports.
Additionally, few more airports were selected for mathematically transforming geo-coordinates of an arbitrarily assumed point each inside those airports from Everest System to WGS-84 by using MADTRAN software. These airports were Tezu, Dibrugarh, Along, Port Blair, Car Nicobar, Tuticorin, Cochin, Agatti, Porbandar, Jaisalmer, Srinagar, Kargil, and Leh. The differences between WGS-84 and Everest coordinates at the above mentioned airports were also examined; however, no GPS observation was made for these airports. The observation results for these airports are summarized and presented in Table 2.
Table 1: Latitudes and Longitudes of Airports where Actual GPS Observations were made at the ARPs, to Show the Everest Coordinates, WGS-84 Actually Observed and WGS-84 Mathematically Converted (Transformed) Coordinates.
| Sl.No. |
Name of Airport |
Everest 3 Co-ordinates |
Precise WGS-84 Co-ord. (GPS Observation) |
WGS-84 Co-ordinates (Mathe. Transformed) |
Difference between
WGS-84 (Transformed) andEverest |
Difference between
WGS-84 (Converted)
and WGS-84 (Precise) |
| Latitude (N) Longitude (E) |
Latitude (N) Longitude (E) |
Latitude (N) Longitude (E) |
Seconds |
Seconds |
| 1 |
DELHI |
28°-34'-07.3" |
28°-34-7.42185" |
28°-34-7.5" |
(+) 0.2 |
(+) 0.07815 |
| |
|
77°-06"-47.9" |
77°-06-43.69343" |
77°-06-43.4" |
(-) 4.5 |
(-) 0.29343 |
| 2 |
AHEMEDABAD |
23°-04'-14.4" |
23°-04'-16.28032" |
23°-04'-16.3" |
(+) 1.9 |
(+) 0.01968 |
| |
|
72°-37'-37.3" |
72°-37'-35.15308" |
72°-37'-35.1" |
(-) 2.20 |
(-) 0.05308 |
| 3 |
KOLKATA |
22°-39"-11.03" |
22°-39'-14.24691" |
22°-39'-13.6" |
(+) 2.57 |
(-) 0.64691 |
| |
|
88°-26'-57.25" |
88°-26'-48.18029" |
88°-26'-47.7" |
(-) 9.55 |
(-) 0.48029 |
| 4 |
MUMBAI |
19°-05'-26.88" |
19°-05'-29.54342" |
19°-05'-30.1" |
(+) 3.22 |
(+) 0.55658 |
| |
|
72°-52'-00.01" |
72°-51'-57.53373" |
72°-51'-57.8" |
(-) 2.21 |
(+) 0.26627 |
| 5 |
HYDERABAD |
17°-27'-08.6" |
17°-27'-12.0076" |
17°-27'-12.4" |
(+) 3.80 |
(+) 0.3924 |
| |
|
78°-27'-49.5" |
78°-27'-44.8952" |
78°-27'-44.7" |
(-) 4.80 |
(-) 0.1952 |
| 6 |
CHENNAI |
12°-59'-36.6" |
12°-59'-41.26623" |
12°-59'-41.7" |
(+) 5.10 |
(+) 0.43377 |
| |
|
80°-10'-37.3" |
80°-10'-32.22512" |
80°-10'-31.8" |
(-) 5.50 |
(-) 0.42512 |
| 7 |
BANGALORE |
12°-57'-03.39" |
12°-57'-07.85655" |
12°-57'-08.4" |
(+) 5.01 |
(+) 0.54345 |
| |
|
77°-39'-56.58" |
77°-39'-52.58417" |
77°-39'-52.2" |
(-) 4.38 |
(-) 0.38417 |
| 8 |
CALICUT |
11°-08'-11.47" |
11°-08'-16.31058" |
11°-08'-17.0" |
(+) 5.53 |
(+) 0.68942 |
| |
|
75°-57'-05.05" |
75°-57'-02.03711" |
75°-57'-01.5" |
(-) 3.55 |
(-) 0.53711 |
| 9 |
COIMBATORE |
11°-01'-44.3" |
11°-01'-49.17249" |
11°-01'-49.9" |
(+) 5.60 |
(+) 0.72751 |
| |
|
77°-02'-34.9" |
77°-02'-31.24667" |
77°-02'-30.8" |
(-) 4.10 |
(-) 0.44667 |
| 10 |
TRICHY |
10°-45'-51" |
10°-45'-56.18154" |
10°-45'-56.7" |
(+) 5.70 |
(+) 0.51846 |
| |
|
78°-42'-58" |
78°-42'-53.93941" |
78°-42'-53.2" |
(-) 4.80 |
(-) 0.73941 |
| 11 |
TRIVANDRUM |
08°-28'-40" |
08°-28'-46.12243" |
08°-28'-47.1" |
(+) 7.10 |
(+) 0.97757 |
| |
|
76°-55'-15" |
76°-55'-12.01753" |
76°-55'-11.3" |
(-) 3.70 |
(-) 0.71753 |
Table 2 Latitudes and Longitudes of Airports to show the Everest Coordinates and WGS-84 mathematically converted (Transformed) Coordinates, by using MADTRAN Software.
| Sl.No |
Name of Airport |
Everest Co-ordinates |
WGS-84 Co-ordinates(Transformed) |
WGS-84 Coord. (Transformed) - Everest Coord. |
| Latitude (N)Longitude (E) |
Latitude (N)Longitude (E) |
Difference inarc-Seconds |
| North- East & East |
| 1 |
TEZU |
27°-54’ N |
27°-54’-1. 7” N |
(+) 1.7 |
| |
|
96°-04’E |
96°-03’-46. 4 E |
(-) 13.6 |
| 2 |
DIBRUGARH |
27°-28’-50”N |
27°-28’-51. 7 ”N |
(+) 1.7 |
| |
|
95°-01’-18”E |
95°-01’-4. 9”E |
(-) 13.1 |
| 3 |
ALONG |
28°-11” N |
28°-11’-1. 4”N |
(+) 1.4 |
| |
|
94°-48’ E |
94°-47’-47” |
(-) 13.0 |
| 4 |
PORT BLAIR |
11°-38’-45”N |
11°-38’-50. 8”N |
(+) 5.8 |
| |
|
92°-48’-06”E |
92°-47’-55. 1”E |
(-) 10.9 |
| 5 |
CAR NICOBAR |
09°-09’ N |
09°-09’-6. 4”N |
(+) 7.6 |
| |
|
92°-49’E |
92°-48’-49. 2”E |
(-) 10.8 |
| South & South- West |
| 6 |
TUTICORIN |
08°-43’-00”N |
08°-43’-6. 2”N |
(+) 6.2 |
| |
|
78°-02’-00”E |
78°-01’-55. 5”E |
(-) 4.5 |
| 7 |
COCHIN |
09°-57’ N |
09°-57’-5. 9”N |
(+) 5.9 |
| |
|
76°-16’ E |
76°-15’-56. 3”E |
(-) 3.7 |
| 8 |
AGATTI |
10°-49’-26”N |
10°-49’-31. 6”N |
(+) 5.6 |
| |
|
72°-10’-37”E |
72°-10’-35. 2”E |
(-) 1.8 |
| West |
| 9 |
PORBANDAR |
21°-38’-59. 1”N |
21°-39’-1. 4”N |
(+) 2.3 |
| |
|
69°-39’-31. 7”E |
69°-39’-31”E |
(-) 0.7 |
| 10 |
JAISALMER |
26°-53’ N |
26°-53’-0. 6”N |
(+) 0.6 |
| |
|
70°-52’E |
70°-51’-58. 7”E |
(-) 1.7 |
| North |
| 11 |
SRINAGAR |
33°-59’N |
33°-58’-58. 1”N |
(-) 0.9 |
| |
|
74°-47’E |
74°-46’-56. 4”E |
(-) 13.6 |
| 12 |
KARGIL |
34°-31’ N |
34°-30’-58”N |
(-) 2 |
| |
|
76°-10’E |
76°-09’-55. 7”E |
(-) 4.3 |
| 13 |
LEH |
34°-08’ N |
34°-07’-58. 2”N |
(-) 0.2 |
| |
|
77°-33’E |
77°-32’-55”E |
(-) 5.0 |