Triangulation কী?

Example 1.9 (One triangle)

A baseline $AB = 8.000\,\mathrm{km}$ is measured; from $A$ the signal $C$ is seen at $70{}^{\circ}$ from the direction of $B$, from $B$ at $60{}^{\circ}$ from the direction of $A$. Then $C = 50{}^{\circ}$ and

No distance but the baseline was walked.

Example 1.10 (The meridian of the metre)

In March 1791 the metre was defined as one ten-millionth of the distance from the pole to the equator along a meridian. To find that distance, two astronomers, Jean-Baptiste Delambre and Pierre Méchain, measured by triangulation the meridian arc from Dunkirk, on the North Sea, to Barcelona, on the Mediterranean — almost seven years of work, Delambre in the northern two thirds, Méchain in the southern third. The two towns differ by $9.646{}^{\circ}$ of latitude; on the modern ellipsoid the arc between their parallels is $1072.2\,\mathrm{km}$ long. Knowing the length of an arc and its difference in latitude gives the length of a degree; the shape of the Earth then gives the whole quarter meridian. Platinum standards of the new metre were made in 1799 and kept in the national Archives.