The basic problem of trigonometry runs somewhat like this:

You stand next to a wide river and need to know the distance across it--say to a tree on the other shore, marked on the drawing here by the letter C (for simplicity, let's ignore the 3rd dimension). How can this be done without actually crossing the river?

The usual prescription is as follows. Stick two poles in the ground at points A and B, and with a tape measure or surveyor's chain measure the distance c between them ("the baseline").

The length c of the baseline, and the two angles A and B, contain all there is to know about the triangle ABC--enough, for instance, to construct a triangle of the same size and shape on some convenient open field. Trigonometry (trigon = triangle) was originally the art of deriving the missing information by pure calculation. Given enough information to define a triangle, trigonometry lets you calculate its remaining dimensions and angles.

Why triangles? Because they are the basic building blocks from which any shape (with straight boundaries) can be constructed. A square, pentagon or another polygon can be divided into triangles, say by straight lines radiating from one corner to all others.

    In mapping a country, surveyors divide it into triangles and mark each corner by a "benchmark", which nowadays is often a round brass plate set into the ground, with a dimple in its center, above which the surveyors place their rods and telescopes (George Washington did this sort of work as a teenager). After measuring a baseline--such as AB in the example of the river--the surveyor would measure (as described here) the angles it formed with lines to some point C, and use trigonometry to calculate the distances AC abd BC. These can serve as baselines for 2 more triangles, each of which provides baselines for two more... and so on, more and more triangles until the entire country is covered by a grid involving only known distances. Later a secondary grid may be added, subdividing the bigger triangles and marking its points with iron stakes, providing additional known distances on which any maps and plans can be based.

    A very readable account, including the story of William Lambton, who founded that survey, can be found in "The Great Arc : The Dramatic Tale of How India Was Mapped and Everest Was Named" by John Keay, 160 pages hard cover, Harper and Collins, Sept. 2000.