Survey and Maps

When people started to move from place to place they memorized the route by different land marks, so that they can retrace back. Then they made rough sketches, which led to modern day maps. For large track of land now we use different equipments to make maps. At the beginning land holdings were measures in a chain and length and breadth is recorded. For large areas angle is also required to be measured, for which theodolite is used, which gives horizontal angle and vertical angle. For detailed surveys plane table, compass, ranging rods and level instrument are used.

The geodetic survey of India (GTS) was conducted for about 5 years at the beginning of 20^th century by survey of India. This is still known as most extensive survey with details like, trees, telephone poles, wells and even weekly markets, in addition to roads, bridges, houses etc. The base line was about 5 miles marked by placing one invar bar against another and accurate distance is marked. Then a series of triangulations was done by using theodolite, and calculations done by seven figure logarithm tables. The details were filled up by plane table and compass.

The height of Mount Everest was done by vertical triangulation by the same method. These maps with slight modification on new constructions are now used in India by engineers, revenue forest officials, and armed forces. The GTS bench marks etched in granite stone are still to be found at the peak of mountains, dak bungalows and railway stations all over India. It is said that the original measurements were checked by modern instruments and the error was a fraction of an inch.

In all maps contours are drawn for same level lines, by which you can find elevations and depressions. This is based on mean sea level, which was taken at Bombay, after observing low tides and high tides through the year.

Comparing the above, the levels marked in the maps of Europe and that of Africa was found to be different near Gibraltar by 200 ft. When panama canal joined pacific with Atlantic oceans, it was found to have so much difference that lock gates were built for the ships to negotiate.

Now aerial photography has taken over without moving on the ground, which was done in Canada and Australia earlier. This is done by taking a series of photos overlapping each other at a specific height. Latest is google which takes photographs from satellites.