Two hun­dred and fifty years of Sur­vey of In­dia

Alive - - Contents - by G.V. Joshi

The con­tri­bu­tion of sur­vey­ors from Sur­vey of In­dia is very im­por­tant and a

ma­jor mile­stone achieve­ment.

To­day, any ad­ven­ture or travel be­gins with col­lec­tion of rel­e­vant maps. But that was not so 250 years ago. In 1757, af­ter their vic­tory in the Bat­tle of Plassey, the English East In­dia Com­pany (EICO) was first granted a za­min­dari by Mir Jaf­far and by 1765 the nom­i­nal Delhi em­peror granted them the De­wani of the subah of Ben­gal.

Events were mov­ing in a man­ner which ul­ti­mately trans­formed the EICO into a king­dom. And they knew al­most noth­ing about their new ac­qui­si­tion.

This made it nec­es­sary for them to draw a map of their new pos­ses­sions and to achieve that they ap­pointed James Ren­nell as their first Sur­veyor Gen­eral in 1767.

A map is a graphic rep­re­sen­ta­tion of ge­o­graph­i­cal fea­tures of an area of the Earth, drawn to scale and usu­ally on a flat sur­face like pa­per or cloth. Globes are maps rep­re­sented on the sur­face of a sphere.

A map is drawn on some scale. The scale refers to the dis­tance be­tween two ob­jects, say tem­ples on the map as com­pared to their po­si­tion on the ground. It is usu­ally con­ve­nient to ex­press the scale by a rep­re­sen­ta­tive frac­tion or pro­por­tion, as 1/50,000, [one me­tre to 50 kilo­me­tres] or (1cm to 500 me­tres). In other words, if the dis­tance be­tween two promi­nent tem­ples or trees is 500 me­tres on the ground, it is shown as one cen­time­tre on the map.

A good map also shows the po­si­tion of an ob­ject in lat­i­tude and lon­gi­tude. The de­ter­mi­na­tion of lat­i­tude and lon­gi­tude was well es­tab­lished by this time. The idea that the Earth is an oblate spher­oid (like an or­ange) was also known.

In other words, the ra­dius along the equa­tor was slightly more than the ra­dius along the poles. But the ac­cu­rate value of the equa­to­rial ra­dius on of Earth and the ques­tion, 'how much more than the po­lar ra­dius', re­mained unan­swered.

The de­ter­mi­na­tion of the dis­tance be­tween two points on Earth sep­a­rated by one-de­gree lat­i­tude gives the sur­vey­ors some idea about the curva-

of the Earth in that re­gion. For a per­fect sphere, it should be the same ev­ery­where. But for an oblate spher­oid, this would be less near the equa­to­rial lat­i­tudes than at the poles.

It is also pos­si­ble to work out the ra­dius of the Earth from this in both di­rec­tions from th­ese data.

Some mea­sure­ments were made by the fa­ther and son team of Cassi­nis in France.

Sur­pris­ingly, the re­sult of that ex­per­i­ment showed that the length of a merid­ian de­gree north of Paris to be 111,017 me­tres, or 265 me­tres shorter than one south of Paris (111,282 me­tres).

This sug­gested that the Earth is a pro­late spher­oid, i.e., one elon­gated at the poles like an egg, with the equa­to­rial ra­dius shorter than the po­lar ra­dius. This was com­pletely at odds with New­ton's con­clu­sions.

In or­der to set­tle the con­tro­versy caused by New­ton's the­o­ret­i­cal deriva­tions and the mea­sure­ments of Cassi­nis, the French Academy of Sci­ences sent two ex­pe­di­tions, one to Peru, South Amer­ica, near equa­tor in 1735 and an­other to La­p­land in the Arc­tic, in 1736.

The Peru ex­pe­di­tion was led by Bouger and La Con­damine and the ob­ject was to mea­sure the length of a merid­ian de­gree. The La­p­land ex­pe­di­tion led by Mau­per­tuis was to make sim­i­lar mea­sure­ments.

Map­ping curves

Both par­ties de­ter­mined the length of the arcs us­ing the well­known method of tri­an­gu­la­tion in sur­vey­ing. The ex­pe­di­tion to La­p­land re­turned in 1737, and Mau­per­tuis re­ported that the length of one de­gree of the merid­ian in La­p­land was 111.95 km. The Peru re­sult was 110.61 km. This proved that New­ton was right. For Paris, some­where in the mid­dle lat­i­tude, the re­sult was 111.21 km.

One of the most im­por­tant work done by Col. Lambton fol­lowed by Ge­orge Ever­est, sur­vey­ors from Great Trig­no­met­ri­cal Sur­vey (GTS) un­der Sur­vey of In­dia was the mea­ture sure­ment of an arc of merid­ian along 780 East lon­gi­tude from Tirunalvelli in Tamil Nadu to Banog Hill near Mus­soorie in Ut­trak­hand.

At that time, the work was jus­ti­fied as part of an at­tempt to pro­vide an ac­cu­rate base for sys­tem­atic to­po­graphic and rev­enue sur­veys, but it was also part of an at­tempt to an­swer one of the thorni­est sci­en­tific prob­lems of the day, the de­ter­mi­na­tion of equa­to­rial and po­lar ra­dius of Earth.

The re­sult of Lambton’s and later Ever­est’s work also con­firmed that New­ton was right. The radii de­ter­mined by Ever­est and his fol­low­ers were used in de­ter­min­ing the shape of the Earth and they are still be­ing used by Sur­vey of In­dia.

Al­though Global Po­si­tion­ing Sys­tem (GPS) is much quicker than the meth­ods used then by Sur­vey of In­dia, the math­e­mat­ics used by GPS would not have been pos­si­ble with­out know­ing the di­men­sions of Earth and there­fore the con­tri­bu­tion of sur­vey­ors from Sur­vey of In­dia is very im­por­tant and should be recog­nised as a ma­jor mile­stone achieve­ment.

The 1870 In­dex of the Great Trigono­met­ri­cal Sur­vey of In­dia.

World po­lit­i­cal map as a globe.

Po­lit­i­cal map of modern In­dia.

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